Curtido (Spicy Cabbage Slaw for Pupusas, etc!)

Ingredients: 1/2 head cabbage, shredded 1 large carrot, grated 1/2 onion, thinly sliced 1/2 cup apple cider vinegar 1/4 cup water 1/2 teaspoon salt 1/2 teaspoon brown sugar 1 teaspoon dried oregano (preferably Mexican) 1/2 to 1 teaspoon red pepper flakes Combine the...