Brown the Ground Beef with the chopped onion. Season with Salt, pepper and garlic powder. Drain fat. In a large mixing bowl, whisk together cream soup, sour cream and milk. Add drained green beans, cheese and hamburger to the large bowl and mix all together. Pour mixture into a greased 9 X 13 pan. Place Tater tots evenly over the entire pan, and spray the top of the tater tots with a little oil to crisp them up. Bake uncovered in a 400 degree oven for 40 minutes. Let hotdish rest for 5 minutes before serving.
You can make ahead and refrigerate or freeze. Allow frozen hotdish to thaw in fridge overnight before baking. When ground beef is cold, bake for 45 minutes to an hour.