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 immediately immerse in ice water for several minutes to stop the cooking process. 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 between your palms.
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.