Cook the beef mixture: Preheat your oven to 375°F (190°C). Heat a large skillet over medium heat and add the olive oil or butter. Add the diced onion and cook for about 2–3 minutes until it begins to soften and become slightly translucent. Stir in the minced garlic and cook for another 30 seconds until fragrant. Add the ground beef and cook while breaking it apart with a spoon or spatula. Continue cooking for about 6–8 minutes until the beef is browned and fully cooked. Drain any excess grease from the pan.
Season the filling: Stir the salt, black pepper, ketchup, yellow mustard, and Worcestershire sauce into the cooked beef mixture. Mix everything well so the flavors are evenly distributed and the beef begins to taste like a classic cheeseburger filling. In a small bowl, whisk together the cream of mushroom soup and milk until smooth. Pour this mixture into the skillet and stir until the beef mixture becomes creamy and well combined. Let it simmer for 2–3 minutes so the sauce thickens slightly.
Assemble the casserole: Lightly grease a 9x13-inch baking dish. Pour the beef and sauce mixture into the dish and spread it out evenly to form the base layer. Sprinkle about 1 cup of shredded cheddar cheese evenly over the beef layer so it melts into the filling while baking.
Add the fries: Arrange the frozen French fries on top of the cheese layer in a single even layer. You can line them up neatly or scatter them across the top, making sure the entire surface is covered. The fries will bake up crispy while the casserole cooks underneath.
Bake the casserole: Place the baking dish in the preheated oven and bake for about 30 minutes. The fries should begin turning golden brown and crispy. Remove the dish from the oven, sprinkle the remaining 1 cup of shredded cheddar cheese over the top of the fries, and return it to the oven for another 10 minutes until the cheese is melted, bubbly, and slightly golden.
Serve: Remove the casserole from the oven and let it rest for about 5 minutes before serving so the layers can settle slightly. Scoop generous portions onto plates and add optional toppings like chopped pickles, diced tomatoes, shredded lettuce, or a drizzle of ketchup and mustard to give it that classic cheeseburger flavor. Serve warm and enjoy this comforting, cheesy, and crispy dinner.