2 cups shredded green cabbage
1/2 cup apple cider vinegar
1/2 cup water, or more as needed
1/2 onion, thinly sliced
1 carrot, grated
1/4 teaspoon ground oregano
1/4 teaspoon red pepper flakes
salt to taste
Pupusa Dough:
3 cups masa harina flour (Mexican corn masa mix)
1 1/2 cups water, or more as needed
1/2 teaspoon salt
Pupusa Filling:
1 cup ricotta cheese
1 cup shredded mozzarella cheese
2 tablespoons heavy whipping cream, or more to taste
1 scallion, finely chopped, or more to taste
cooking spray

Bring a large pot of lightly salted water to a boil.
Add cabbage and cook uncovered until slightly softened, about 3 minutes.
Drain in a colander and immerse in ice water for several minutes to stop cooking.
Drain.
Mix cabbage, apple cider vinegar, water, onion, carrot, oregano, red pepper flakes,
and 1 pinch salt together in a large bowl.
Refrigerate until flavors combine, 8 hours to overnight.
Combine masa harina, water, and 1/2 teaspoon salt together in a large bowl;
knead until a smooth, moist dough forms.
Add water if dough cracks when you press down on it.
Let dough rest, 5 to 10 minutes.
Stir ricotta cheese, mozzarella cheese, heavy cream, and scallion together in a bowl to make a paste.
Divide dough into 8 balls.
Press your thumb into the center of each ball to form an indentation.
Fill indentations with ricotta cheese paste.
Pinch edges together around the filling; flatten and smooth into round 1/4 inch-thick patties.
Grease a skillet with cooking spray; preheat over medium heat.
Cook pupusas in batches until browned, 2 to 3 minutes per side.
Drain cabbage mixture and serve alongside pupusas.