Brown beef and onion together until done, drain off any grease (I brown my beef ahead of time and store it in the freezer to make this quicker). Place ground beef mixture in bottom of 8x8 baking dish. Top with frozen tater tots. In small bowl stir together cream soup and milk. Pour over tater tots. Cover with foil and bake at 350 for thirty minutes.
A lot of recipes don't have you covering it and I used to do mine that way but I've found it is even better if you cover it up a bit. If you'd like your tater tots nice and browned, uncover the last ten minutes of cooking.