In a large skillet cook bacon until crispy; drain and cool on paper towels; crumble and set aside reserving about 1 tablespoon of bacon fat in the skillet.
Preheat oven to 400F.
Add the onion to the skillet and cook over medium-high heat until onion starts to brown. While cooking scrap any dripping off the bottom of the pan. Add the burger and continue cooking breaking up the burger, and cooking until no longer pink inside; drain any excess fat. Add the ketchup and mustard and mix until combined. Spoon the meat mixture into a 9×9 casserole dish lightly sprayed with cooking spray.
Sprinkle the bacon on top of the burger and ½ of the cheese. Arrange the tater tots over the top and bake for 30 – 35 minutes or until tater tots are nicely browned.
Remove the casserole from the oven and sprinkle the rest of the cheese over the top. Bake for an additional 5 minutes or until cheese is melted – serve hot.