Begin by thoroughly rinsing your cabbage, carrots, onion, and jalapeño (if using) to remove any dirt or residue. Using a sharp knife or mandolin, finely shred the cabbage and carrots. Thinly slice the onion and jalapeño. The thinner the cuts, the quicker the fermentation and the better the flavor development. Place all of the shredded and sliced vegetables into a large, non-reactive mixing bowl (glass, ceramic, or stainless steel is ideal). Add the oregano and red pepper flakes (if using), and mix the vegetables with your hands until evenly combined.

Sprinkle the kosher salt over the vegetable mixture. Let it sit for 5 to 10 minutes to start drawing out moisture. Then, with clean hands, begin massaging the salt into the vegetables. This process helps break down the cell walls and encourages the cabbage to release its natural juices. Continue massaging for about 5 to 10 minutes, or until the mixture has significantly softened and there is a visible pool of liquid (brine) collecting at the bottom of the bowl. The brine is essential for the fermentation process, as it creates an anaerobic environment that supports the growth of beneficial bacteria.

Once your vegetables have released enough brine, begin transferring the mixture into a clean quart-sized mason jar (or a fermentation crock, if you prefer). Use your hands or a wooden spoon to press the mixture firmly down into the jar as you go, helping to eliminate air pockets and encouraging the vegetables to become fully submerged in the brine. Pour any remaining brine from the bowl over the top of the packed vegetables. Leave at least 1 inch of headspace at the top of the jar to allow room for bubbling during fermentation.

To keep the vegetables safely submerged under the brine, place a fermentation weight on top if you have one, or use a small ziplock bag filled with water as an improvised weight. Screw a lid on loosely if using a regular jar lid, or use an airlock lid if you have one. You can also use a cheesecloth with a rubber band, though this option is more aromatic in the cupboard. It’s important that gases can escape, so don’t overtighten. If using a regular lid, you’ll need to “burp” the jar once or twice a day during active fermentation to release built-up pressure.

Place the jar on a plate or shallow bowl (to catch any potential overflow) and store it in a cool, dark place—ideally between 65–72°F. Allow it to ferment for 2–5 days. After the 2nd day, you can begin tasting it daily until it reaches your preferred level of tanginess. Most curtido ferments well between 3–7 days, depending on the temperature and your flavor preference. The longer it ferments, the tangier and softer it will become. If any foam or harmless white yeast (kahm yeast) appears, just skim it off—it’s normal.

Once it’s fermented to your liking, remove the weight, screw the lid on tightly, and store the curtido in the refrigerator. This will slow fermentation and preserve its flavor and texture. It will continue to develop slightly in the fridge but will last several months. Serve chilled or at room temperature with pupusas, tacos, grilled meats, or as a tangy side dish to nourish your gut and enhance your meals.