Preheat the Oven: Set your oven to 350°F (175°C) and grease a 9×13-inch baking dish to prevent sticking.
Cook the Beef: In a large skillet over medium heat, cook the ground beef and diced onion until the meat is fully browned and the onions are soft and translucent, about 6–8 minutes. Drain any excess fat. Season with garlic powder, salt, and pepper to taste. This forms the savory, meaty base of your casserole.
Make the Sauce: In a bowl, whisk together the cream of mushroom soup, milk, ketchup, and yellow mustard until smooth. This creamy, tangy mixture brings all the cheeseburger flavors together.
Assemble the Base: Pour the sauce mixture into the skillet with the cooked beef and stir well to combine. Spread the beef mixture evenly in the prepared baking dish.
Add the Cheese: Sprinkle the shredded cheddar cheese over the beef layer. This will melt and mingle with the sauce while baking, creating a cheesy, gooey layer beneath the fries.
Top with Fries: Arrange the frozen French fries in an even layer over the cheese. You can overlap slightly, but try to spread them out so they bake evenly and get crispy.
Bake: Place the dish in the preheated oven and bake uncovered for 30–35 minutes, or until the fries are golden brown and crisp. For best results, rotate the dish halfway through baking to ensure even cooking.
Optional Topping Step: For an extra cheesy finish, sprinkle more shredded cheese over the top of the fries and return to the oven for an additional 5 minutes, or until the cheese is melted and bubbly.
Cool & Garnish: Let the casserole cool for about 5–10 minutes before serving. Garnish with classic burger toppings like sliced pickles, fresh diced tomatoes, or shredded lettuce for added crunch and freshness.
