Preheat oven to 350 degrees. Spray a 9x13 baking dish with nonstick cooking spray.
In a large skillet brown up ground beef with onion until beef is fully cooked and onions are soft. Drain grease.
Add garlic and cook for an additional one minute.
Add in corn, tomatoes, chilies, taco seasoning and 2 cups of Doritos, broken into small pieces. Stir for 3-4 minutes. Turn off burner.
Stir in sour cream and 1/2 of the Monterey jack cheese.
Spread 1 1/2 cups of broken Doritos into the bottom of prepared baking dish.
Spread 1/2 of the meat mixture over the Doritos carefully.
Sprinkle the other 1/2 cup of Monterey jack cheese over meat mixture.
Sprinkle 1 1/2 cups of broken Doritos evenly over meat mixture and cheese.
Place the remaining meat mixture on top and smooth evenly.
Layer another layer of Doritos from bag on top of mixture. Sprinkle with remaining 1 cup of Monterey jack cheese.
Lightly spray a sheet of foil with nonstick cooking spray. Place spray side down on casserole and seal edges.
Bake for 30 minutes. Remove foil and bake an additional 10-15 minutes until cheese has melted. Sprinkle with green onions if desired.
