Toss lettuce, onion, and tomatoes, well. Place in the bottom of a 9x12 cake pan. Mix cottage cheese and taco seasoning well. Pour cottage cheese mixture over lettuce, onion and tomatoes. Put all of the cheddar cheese on top evenly.
Put in fridge covered with plastic wrap for about 30 min.
Take the salad out of the fridge, take off the plastic wrap and place Doritos, as many as you like around the sides of the salad.