Prepare the curtido by combining shredded cabbage, grated carrots, and sliced onion in a large bowl.

Bring water, vinegar, oregano, red pepper flakes, and salt to a boil and pour over the cabbage mixture. Let cool and refrigerate for at least 2 hours.

Make the salsa roja by combining tomato sauce, water, cilantro, green bell pepper, onion, crushed garlic, bouillon cube, and salt. Cook until vegetables are soft, about 20 minutes.

Prepare pupusa dough by mixing masa harina, salt, and warm water until a soft, moist dough forms.

Divide the dough into 15 balls and form each into a thick disk with a depression in the center.

Fill each disk with cheese or other fillings, seal the edges, and flatten into a ¼-inch thick patty.

Cook the pupusas on a hot griddle for 3-4 minutes per side until golden brown spots appear.

Serve immediately with curtido and salsa roja.