2. Combine 1/2 cup cheese and next 8 ingredients (through onions) in a large bowl, stirring just until moist. Pour into a 13 x 9 baking dish coated with cooking spray.
3. Bake for 15-20 minutes or until set. Pierce entire surface liberally with a fork; pour enchilada sauce over top. Top with meat of choice; sprinkle with remaining cheese. Bake for 15 minutes or until cheese is melted. Remove from oven; let stand for 5 minutes. Cut into 8 serving pieces and top each with 1 tablespoon of sour cream and sprinkle with chopped tomatoes if desired,