Brown ground beef and onions; drain off fat. Add salt, pepper and cooked beans.
In a medium saucepan, combine tomatoes, chilies, garlic, cumin, chili powder, oregano and
tomato sauce. Bring to a boil; lower heat, cover and simmer for 10 minutes.
Spread a thin layer of sauce on the bottom of a 6x10-inch baking pan. Top with 3 tortillas,
overlapping as necessary.
Spread half of bean/beef mixture over tortillas. Pour sauce over to cover. Sprinkle with half the cheese.
Repeat layers with remaining ingredients, ending with a layer of cheese.
Bake 30 minutes at 350°. Cut into 6 rectangles. Top with low-fat sour cream, plain yogurt or salsa, if desired.