In a skillet, cook the ground beef and chopped onion over medium heat until the beef is browned. Drain any excess fat.
In a large bowl, mix the cream of mushroom soup, milk, ketchup, mustard, salt, and pepper. Add the cooked beef and onion mixture and stir to combine.
Spread the beef mixture evenly in a baking dish. Top with a layer of frozen French fries.
Bake in the preheated oven for 30 minutes. Remove from the oven, sprinkle shredded cheddar cheese on top, and bake for an additional 15 minutes, or until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving.
Notes:
This casserole is perfect for a family dinner or a potluck. You can also customize it by adding your favorite burger toppings like pickles or bacon.