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Combine the sliced cabbage, grated carrots, and sliced onion in a large mixing bowl.
In a separate bowl, whisk together the apple cider vinegar, water, salt, oregano, and red pepper flakes to create the dressing.
Pour the dressing over the cabbage mixture and toss until the vegetables are well coated.
If using, add the sliced jalapeños or other hot peppers to the mix and stir to combine.
Transfer the mixture to a jar or airtight container, pressing down the cabbage so it's submerged in the liquid as much as possible.
Let the curtido ferment at room temperature for at least 2 hours. For a stronger flavor, you can leave it overnight or for up to 48 hours.
Once fermented to your liking, refrigerate the curtido to stop the fermentation process. It will keep in the refrigerator for up to two weeks.