Chicken Tortilla Casserole

12 ounces boneless, skinless chicken breasts,
    cut into bite-size pieces
1 cup (4 ounce) shredded Cheddar cheese
3/4 cup chopped onion
1 tablespoon butter
1 (15 ounce) can tomato sauce
2 (4 ounce) cans mild chopped green chiles
1 tablespoon chili powder
1 teaspoon cumin
1 cup sour cream
1 1/2 cups (6 ounces) shredded Monterey jack cheese, divided
5 (10-inch) flour tortillas
2 cups whole kernel corn, drained

Lightly grease a 2-quart casserole dish.

In a large skillet over medium heat, saut