Preheat oven to 350 degrees F. In a large pot, combine ground beef and chopped onion and brown until done. Strain off grease. Add cream of mushroom soup, cream of chicken soup and enchilada sauce, heat through. Season sauce to taste.
Oil a 12 x 18 pan. Layer sauce, tortillas and cheese into the pan, repeat.
Bake for 30 minutes or until the cheese is melted.