Kimchi Smash Burger in Madrid: the only one with kimchi fermented in-house | STILLROOM Jerónimos

Kimchi Smash Burger in Madrid: STILLROOM’s unique take

In the heart of the Los Jerónimos neighbourhood, just five minutes’ walk from the Prado Museum, STILLROOM has established itself as one of Madrid’s most innovative gastro cocktail bars. Its venue, on Calle Moreto 9, brings together a menu of signature cocktails with a food offering that is not afraid to experiment with flavours from around the world.

The Kimchi Smash Burger is the kitchen team’s boldest move to fuse Korean tradition with the American technique of the smash burger. What makes this burger truly unique on the Madrid food scene is its kimchi, fermented in-house for 24 to 48 hours — a process no other venue in the capital replicates with this level of dedication.

The smash burger phenomenon has taken over Madrid in recent years, with dozens of venues offering their own take on the burger flattened onto a hot griddle. STILLROOM, however, has found its point of difference in the addition of house-made kimchi, elevating what looks like a casual dish into a sophisticated dining experience. The house fermentation of the kimchi brings an acidic, umami complexity that no industrial product can replace, turning every bite into a sensory journey between Seoul and Texas.

Anatomy of the bite: layer by layer

STILLROOM’s Kimchi Smash Burger is a meticulous build in which every element has a specific role to play in the dish’s overall balance:

• Brioche bun: soft, lightly sweet, with a golden crust that gives structure without weighing it down.
• First smash patty (80g): high-quality beef flattened at high heat to create an intense, caramelised crust.
• Melted American cheese: melts over the hot patty, adding creaminess and a characteristic salty note.
• Second smash patty (80g): a second patty that doubles the meaty intensity and builds layers of crisp texture.
• In-house fermented kimchi: the star element, with its crunchy texture and its acidic, spiced, deeply umami profile.
• Onion caramelised in beef fat: slow-cooked in the fat rendered by the patties themselves, adding sweetness and depth.
• House hot sauce: a touch of heat that lifts the whole without overpowering the other flavours.
• Bottom bun: which acts as a stable base to hold all of the above.

This architecture of the bite is not arbitrary: the contrast between the creaminess of the cheese, the acidity of the kimchi, the sweetness of the onion and the crunch of the crust creates a multidimensional experience in which no single flavour dominates the others. Every layer brings texture, temperature and flavour, in a balance that only works when all the elements are present.

Why kimchi fermented in-house: the technique

Lactic fermentation is the process that transforms raw napa cabbage into the kimchi we know so well. This traditional Korean method harnesses the lactic acid bacteria naturally present on the vegetables to turn them into a living food, rich in probiotics, with a flavour profile that keeps evolving over time.

STILLROOM’s kimchi follows a rigorous process that starts with selecting fresh napa cabbage, salted in a 3% brine to draw out excess water and create the right environment for fermentation. The cabbage is then mixed with gochugaru (Korean chilli powder), freshly grated ginger, crushed garlic and strips of daikon. Some batches include Korean fish sauce to add umami depth, though this is left to the chef’s judgement depending on the season.

The fermentation itself happens in two stages: first at room temperature for 24 to 48 hours, when bacterial activity is most intense and the kimchi develops its most complex, slightly fizzy flavours; it is then moved to the fridge for a slow fermentation that can run on for several weeks, losing some intensity but gaining complexity.

The result is a kimchi with a complex acidity, a heat that warms rather than burns, and a deep umami that acts as a natural enhancer for the flavour of the meat. An industrial, pasteurised, shelf-stable product could never offer this kind of aromatic evolution. That is why STILLROOM makes its kimchi in small batches, ensuring that every burger that goes out to the table carries a living product at its peak moment for eating. If you would like to dig deeper into similar fermentation techniques applied to cocktail making, take a look at our cocktail and mixology glossary.

The smash technique: Maillard science at 280 degrees

The smash burger owes its characteristic texture to the Maillard reaction, a chemical process that occurs when amino acids and reducing sugars interact at temperatures above 140 degrees Celsius. At STILLROOM, the griddle reaches 280 degrees Celsius, a temperature that maximises this reaction and creates the golden, crisp crust that defines a well-executed smash burger.

The technique involves placing a ball of minced beef (in this case, two 80-gram portions for the double version) onto the hot griddle and flattening it immediately with a metal spatula. This flattening has a double function: on one hand, it increases the surface area in contact with the heat, creating more area for the Maillard reaction; on the other, it presses some of the internal fats out to the surface, where they help form the crust.

Cooking time is short but precise: around 90 seconds per side. This is enough time to develop the characteristic browning without the meat losing its interior juiciness. The contrast between the crisp exterior and the juicy interior is what sets a smash burger apart from a thicker, more traditional burger, where heat penetrates slowly and can leave a drier texture.

The griddle temperature is critical: below 250 degrees, the Maillard reaction does not occur with sufficient intensity, and the meat comes out pale and lacking in character. Above 300 degrees, the exterior burns before the interior reaches the right temperature. STILLROOM’s 280 degrees represents the sweet spot where science and gastronomy meet.

How to make it at home: an accessible version

Although recreating STILLROOM’s Kimchi Smash Burger at home takes some planning ahead, especially for the kimchi fermentation, the result can be remarkably satisfying. Here is a guide for two servings:

• 400 grams of high-quality minced beef (at least 20% fat)
• 2 brioche burger buns
• 2 slices of American cheese
• 200 grams of napa cabbage for home-made kimchi
• 2 tablespoons of gochugaru
• 1 piece of fresh ginger (about 3 centimetres)
• 4 cloves of garlic
• 1 small daikon
• 3% brine (30 grams of salt per litre of water)
• 1 medium onion
• Hot sauce, to taste
• Salt and freshly ground black pepper

The process needs two days’ lead time for the kimchi. First, cut the napa cabbage into manageable pieces and submerge them in the brine for 2–3 hours. Drain well and mix with the gochugaru, grated ginger, chopped garlic and julienned daikon. Pack the mixture tightly into a glass jar and leave it to ferment at room temperature for 24–48 hours. After that, store it in the fridge.

For the burger, divide the beef into four 100-gram portions — two per burger — and shape into loose balls. Heat a pan or griddle until very hot (aim for 280 degrees if your equipment allows it). Place each ball of meat down and flatten it immediately with a spatula. Cook for 90 seconds per side, add the cheese in the last 30 seconds and let it melt with the residual heat.

Build the burger with the toasted brioche bun, drained kimchi, onion caramelised in the beef fat and hot sauce. The result will get you close to the STILLROOM experience, although professional technique and the ingredients we source remain irreplaceable.

Where to try our Kimchi Smash Burger in Madrid

STILLROOM is on Calle Moreto 9, 28014 Madrid, in the heart of the Los Jerónimos neighbourhood, barely five minutes’ walk from the main entrance of the Prado Museum. Its prime location makes it the perfect spot for lunch or dinner before or after a cultural visit, or simply to discover one of the most innovative dining concepts in central Madrid.

The venue opens Wednesday and Thursday from 11:00 to 19:00, Friday and Saturday from 11:00 to 15:00 and from 19:00 to 01:30, and Sunday from 11:00 to 23:00, so the Kimchi Smash Burger can be enjoyed at lunchtime as well as at dinner on Friday, Saturday and Sunday, with the small hours ahead of you on Friday and Saturday for a last drink. We are closed on Monday and Tuesday.

To guarantee availability, especially at weekends when demand is higher, STILLROOM recommends booking a table through its online booking system. You can also call 619 21 36 16 for enquiries or large groups.

Our pairing recommendation for the Kimchi Smash Burger is the Highball Martini Tomate, whose umami profile perfectly complements the burger’s complex flavours. Alternatively, the smoky mezcal Punch brings a smoky contrast that balances out the kimchi’s acidity. Both cocktails are part of a menu of 17 signature creations that use advanced techniques such as fat washing and lacto-fermentation.

Whether you are a smash burger lover looking for new experiences or a curious food fan ready to discover unexpected flavours, STILLROOM is waiting for you in Los Jerónimos for an experience that combines technique, tradition and creativity. Book your table and discover why the Kimchi Smash Burger has become one of the most talked-about burgers in Madrid.