Preheat oven to 375 degrees F.
In a large skillet over medium heat, cook the ground beef and onions together until the beef is browned and cooked through.
In a medium bowl, mix together the cream of mushroom soup, water, and Rotel Tomatoes.
Spread half of the Doritos in the bottom of a 9x13 inch baking dish.
Layer the ground beef and onion mixture over the Doritos.
Pour the soup mixture over the beef and sprinkle with the grated cheese.
Top with the remaining Doritos.
Bake for 30 minutes, or until the cheese is melted and bubbly.
Serve hot.