Chicken Enchilada Casserole

1 small onion, chopped
2 tablespoons margarine
1 can cream of mushroom soup
1 can cream of chicken soup
1 cup sour cream
1 (4 ounce) can chopped green chiles
8 chicken breast halves, cooked and quartered
12 corn tortillas
1 1/2 cups Cheddar cheese

Saut