Set oven to 350 degrees F. Chop onion or slice thin according to preference. Brown hamburger together with sausage in pan. When no pink, drain if necessary.
Spread hamburger/sausage mixture on bottom of 9 x 13 casserole dish. Add layer of onion. Spread cheese evenly over that layer. Neatly lay out tater tots over layer of cheese. Mix together can of mushroom soup and, using soup can, a can of milk. Pour this evenly over the tater tot layer.
Bake in preheated oven for 1 1/2 hours. When top is browned, remove from oven. Let cool for about 5-10 minutes to give it time to set up then cut in squares and enjoy!