Tip: Another recipe with the exact-same name "Mexican Chicken Casserole" ranks higher.
In a large bowl, combine cream of mushroom soup, cream of chicken soup, Rotel, chicken, velveeta, rotini pasta and onion. Mix well. In a 13x9 baking dish, spread a layer of Doritos on the bottom of the dish. Next, pour half of the soup mixture over the top of the doritos. Put 1 cup of cheddar cheese on top of that. Top with another layer of Doritos. Pour the rest of the soup mixture on top of the Doritos. Top with the remaining doritos and the remaining cheddar cheese. Cover with foil. Bake at 350 for about 20 minutes. Remove foil. Bake an additonal 5 - 10 minutes until cheese is all the way melted. Enjoy!!