In a large skillet brown beef and onions until no pink remains, about 10 minutes. Drain the fat. Season with salt, pepper, and garlic powder. Place meat mixture in the bottom of a 9 x 13 inch pan or two 8 inch pans or 2 9 inch pie plates.
In a mixing bowl, whisk together cream soup, sour cream, and milk. Pour over hamburger.
Scatter green beans over top of the soup mixture.
Scatter shredded cheese over green beans.
Refrigerate or freeze until ready to bake. Allow frozen casserole to thaw in the fridge overnight before baking. When ready to bake, preheat oven to 350 degrees. Arrange tater tots on top of the cheese. If you are taking it straight from the fridge, place the pan in the oven right away so it can warm up with the oven. Bake until mixture is bubbly and the tots are golden brown, about 45 minutes to an hour if coming from the fridge, or 40 minutes if it's baked while the beef is still warm.