Preheat oven to 350 degrees.
In a large skillet, saute the beef with the onion until the beef is brown and the onion translucent. If you use a lean meat you will need to add a tablespoon of oil or butter.
Add the minced garlic and saute for another minute.
Add the taco seasoning, salsa, cream of mushroom soup, sour cream, and beans.
Open the bag of Doritos (so that it won't pop when you crush them) and crush the Doritos into pieces. Don't crush them too small, however.
In a sprayed 9 by 13 pan, spread 1/2 of the crushed Doritos and top with 1/2 of the beef mixture. Repeat.
Top with cheese.
Bake 15 minutes, covered with foil, or until the cheese is melty. If desired, turn the oven to broil and broil the casserole, uncovered for a couple of minutes to brown.
Serve with lettuce, tomato, black olives, and sour cream.