Salvadoran Pickled Cabbage Slaw

Name: Salvadoran Pickled Cabbage Slaw
Cuisine: American, Latin American
Ingredients: 1/2 medium green cabbage, finely shredded (about 4 cups); 1 medium carrot, grated; 1/2 medium white onion, thinly sliced; 1 cup boiling water; 1 cup distilled white vinegar; 1 tsp dried oregano (preferably Mexican oregano); 1/2 tsp crushed red pepper flakes (optional, for heat); 1 tsp sugar; 1 tsp kosher salt; Freshly ground black pepper to taste