For sauce, wilt onion in hot oil. Add tomatoes, oregano, salsa, and salt. Cook for 15 or twenty minutes. Set aside. Fry tortillas lightly in oil, put some of the sauce and a piece of cheese on each one and roll. Arrange in a buttered shallow glass baking dish. Pour remaining sauce over tacos, top with cheese and spoon sour cream over all. Bake at 160ºC . for 25 to 30 minutes. Serves 6 to 12.