Begin by preparing the vegetables. Use a sharp knife or mandolin to finely shred the cabbage and thinly slice the onion. Grate the carrot using a box grater or food processor. Place all the vegetables in a large, heatproof mixing bowl.

Pour 1 cup of boiling water over the vegetables in the bowl. Stir gently with tongs or a wooden spoon to evenly distribute the heat. Let the mixture sit for 5 minutes to slightly soften the cabbage and onions, enhancing their texture for pickling.

Drain the vegetables thoroughly using a colander, pressing lightly with a spoon to remove excess water. Return the drained vegetables to the mixing bowl and set aside.

In a separate small bowl, whisk together the vinegar, sugar, salt, oregano, and crushed red pepper flakes (if using). Stir until the sugar and salt are fully dissolved, ensuring an evenly balanced brine.

Pour the prepared brine over the vegetables in the mixing bowl. Toss gently but thoroughly to ensure every piece is coated in the pickling liquid. Adjust seasoning with black pepper to taste.

Transfer the mixture to a clean glass jar or container with a lid. Press the vegetables down firmly to submerge them in the brine, leaving a little space at the top for expansion during fermentation.

Seal the jar tightly and let the curtido rest at room temperature for 2 hours to develop flavour. For a deeper tang, refrigerate the jar and let it pickle for 24 hours or up to 3 days. Shake or stir occasionally to redistribute the brine.

Serve the curtido chilled or at room temperature as a vibrant side dish. Traditionally, it pairs beautifully with pupusas or other Salvadoran dishes. Garnish with a sprinkle of fresh oregano leaves if desired for an authentic touch. For a spicier kick, serve with additional hot sauce on the side.