Make the Curtido:

1. Make the curtido at least 24 hours before you want to serve the pupusas.
2. Heat the vinegar, water, salt, and brown sugar until the sugar is melted. Cool the mixture slightly and pour the warm mixture over the vegetables. Add red pepper flakes and oregano and stir thoroughly to combine.
3. Store the curtido in the refrigerator in a large bowl covered with plastic wrap or in mason jars. The top of the cabbage mixture should be submerged under the liquid. If you need to, you can add a little more warm water to cover.

Make the Pupusas:

1. Make the dough. Mix the masa harina, salt, queso fresco, and warm water to form a soft dough. Start with 1 1/2 cups of warm water. If the dough is too dry, add more water tablespoon by tablespoon. The dough is all about feel. Your dough should be soft, but not sticky. The dough should not be cracking when you form it into a ball. When you get the right consistency, knead the dough for a minute and then cover with a clean, damp towel. Let rest 15 minutes.
2. Form the pupusas. Grease your hands with a little bit of oil and form the pupusas into a golf-sized ball. Then, pat it out to form a disc. (See photos above for reference). Place 1 1/2 tablespoons of the shredded mozzarella in the center of the disc, and carefully pinch the dough around the cheese to form into a rough ball again. Then flatten into the final disc, about 4″ wide.
3. When forming the pupusas, store the already-shaped pupusas under a damp cloth to avoid drying out.
4. Heat a large non-stick or cast-iron pan over medium heat for 10 minutes. Lightly grease the pan with oil and cook the pupusas for 4-5 minutes per side. Do not crowd the pupusas, and cook until they form rich brown charred spots.
5. Serve the pupusas hot with curtido and oregano.