Place the flour and vegetable oil in a medium-sized pot over medium heat. Cook and stir until fragrant like pie crust, about 1 minute. Add the salt, chili powder, and paprika, and stir to incorporate. Add 1 cup of water and stir. When the sauce becomes thick, add the next cup of water. Repeat until the entire quart of water has been incorporated into the sauce.
Preheat the oven to 350°F.
Heat the corn tortillas in a small skillet with a little vegetable oil, or in the microwave, sprinkled with a little water and wrapped in a clean dish towel.
Spread a thin layer of sauce in the bottom of a baking dish. Place 1 1/2 tablespoons of cheese in the middle of each corn tortilla and roll them up. Line up the rolled tortillas in the baking dish with the “seams” facing down. Cover the tortillas with more sauce and top with any remaining cheese.
Bake in the preheated oven until the cheese melts, about 15 minutes. Remove from the oven and sprinkle with chopped onion if desired.
