Chiles Rellenos Casserole

1 tablespoon butter or margarine
1 cup chopped onions
2 (4.5 ounce) cans chopped green chiles, drained
1 1/2 cups shredded Cheddar cheese
1 1/2 cups shredded jack cheese
3 eggs
3/4 cup sour cream
1/4 to 1/2 teaspoon crushed red pepper flakes
1 cup chopped seeded tomato or salsa

Preheat oven to 350 degrees F. Grease an 8- to 9-inch square pan.

Melt butter or margarine in skillet. Add onions, stirring until crisp tender. Stir in chiles. Spoon chile mixture into pan, then sprinkle with cheeses.

In a small bowl, beat eggs slightly. Beat in sour cream and pepper flakes. Spoon egg mixture over cheeses and sprinkle with tomato or salsa. Bake for 35 to 45 minutes or until tests done.