Preheat oven to 350 degrees. Brown hamburger and onion together. Drain. Add taco seasoning, enchilada sauce, and tomatoe sauce. Let simmer.
In a 9 x 13 casserole dish, lay out two large tortilla shells to cover bottom. Spread a layer of refried beans on shells. Then put a layer of hamburger mixture, followed by a layer of chedder cheese. Repeat this 2 more times and end up with two shells on top with a layer of cheese.
Bake for about 45 minutes until all cheese is melted and the shells start turning a little brown on the edges. Let cool about 15 minutes.
Cut in slices. Serve with picante sauce, black olives, shredded lettuce, sour cream, etc.