Preheat the oven to 350°F (175°C). In a skillet, cook the ground beef and diced onions over medium heat until the beef is browned and onions are soft.
Drain any excess fat, then season with garlic powder, salt, and pepper. Stir well and set aside.
In a bowl, mix the cream of mushroom soup, milk, ketchup, and mustard until smooth. Add this mixture to the beef and stir to combine.
Transfer the beef mixture into a 9x13-inch baking dish. Spread it evenly and top with a generous layer of shredded cheddar cheese.
Spread the frozen French fries evenly over the cheese layer. No need to thaw them.
Bake the casserole for 30-35 minutes, or until the fries are golden and crispy. For extra cheese, sprinkle some on top during the last 5 minutes and let it melt.
Let the casserole cool slightly, then garnish with optional toppings like diced pickles, chopped tomatoes, or lettuce for extra crunch and freshness.