Heat 4 cups of water until very hot, but not boiling. Place the shredded cabbage in a strainer and pour the hot water over it. Drain well.
Transfer cabbage to a large bowl and add carrots, onion, jalapeño, oregano and salt. Toss to combine.
Pour in the vinegar and ¼ cup water over the vegetables, tossing well to coat everything.
Stuff the curtido into one or two wide-mouth jars, pressing down hard with a wooden spoon until the vegetables are mostly submerged in liquid.
Cover the jar(s) and chill for at least 1 hour, or overnight; the longer the slaw sits, the more flavourful and juicy it will be.
Curtido will keep for up to 2 weeks in the refrigerator.
