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

Heat 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 onion until limp and golden. Return beef to skillet and add tomatoes, chili sauce, salt and pepper. Dip tortilla quarters in taco sauce, covering both sides. Cover bottom of a buttered 3-quart casserole with half the tortilla pieces, slightly overlapping. Spread beef mixture over tortillas. Sprinkle beef with 1 1/2 cups of the cheese. Layer remaining tortilla pieces on top and spread with sour cream. Sprinkle on the spinach and top with remaining cheese. Bake, covered, for 30 minutes, uncovering for the last 15 minutes.

If re-heating, have the casserole at room temperature and bake at 375 degrees F, uncovered, until hot and bubbly.