Mexican-Style Beef Casserole

1 1/2 pounds lean ground beef
2 tablespoons vegetable oil
1 medium onion, chopped
1 cup chopped tomatoes or
    1 (8 ounce) can tomato sauce
1/3 cup chile sauce
Salt, to taste
Freshly ground pepper, to taste
12 corn tortillas, cut into quarters
1/2 cup red taco sauce
3 cups shredded Monterey Jack cheese
1 cup sour cream
2 (10 ounce) packages frozen chopped
    spinach, thawed and well drained

Preheat oven to 375 degrees F. In a large skillet brown the beef. Pour off any fat and remove beef; set aside.

Add oil to skillet and saut