Pre-heat oven to 350. Brown 1 lb. hamburger meat in large skillet. Add onions, salt and pepper. When done, drain and return to skillet. Add 1 can of rotel and 1 can of cream of mushroom soup. Let simmer on low-med heat for 20 minutes. Meanwhile, lay doritos on bottom of 13x9" baking dish. Pour mixture over doritos. Cover casserole with cheese and put in oven and bake until cheese melts.