Preheat your oven as directed on the French fries package so it’s ready to go when you are. Check the temperature and recommended bake time on the bag and set your oven accordingly, allowing it to fully preheat before you add the casserole. A properly preheated oven helps the fries crisp up on top and ensures even cooking, so avoid placing the dish in the oven too early.
In a skillet over medium heat, sauté the ground beef and diced onion until the meat is nicely browned and the onion has softened. Break the beef up into small crumbles as it cooks so it cooks evenly and spreads well in the baking dish. Stir regularly and cook until there is no pink left in the beef and the onions look translucent and tender, usually about 6–8 minutes depending on your stove and pan. If there is excess grease in the skillet once the meat is cooked through, carefully spoon it off or tilt the pan and drain it away to prevent the casserole from becoming too greasy. Don’t forget to season with salt and pepper to bring out those delicious flavors, adding a little at a time and tasting as you go so you don’t over-salt the mixture.
Stir in the mustard and ketchup, making sure everything gets well mixed and the beef is evenly coated. The mixture should look saucy but not soupy, with the ketchup and mustard clinging to the crumbled meat and onions. This is where all those classic cheeseburger flavors come together, so take a moment to mix thoroughly, scraping along the bottom and sides of the skillet. Let the mixture cook for another minute or two, just until it’s heated through and slightly thickened, which helps the flavors meld and prevents excess liquid from pooling in the baking dish.
In a baking dish, spread out your cooked beef mixture in an even layer, pressing it gently so it covers the bottom of the dish from corner to corner. This creates a flavorful base that will support the fries. On top of the beef, arrange a generous layer of frozen French fries, spreading them out as evenly as possible so they cover the meat completely. You can keep them in a single layer or slightly overlapping, depending on the size of your dish, but try not to pile them too thickly in one spot, as that can keep them from crisping. Finish it all off with a good sprinkle of shredded cheese across the top, distributing it evenly so every portion gets some melty, cheesy goodness. The cheese will melt over the fries as the casserole bakes, mimicking the effect of a cheesy loaded burger and fries.
Pop the dish in the oven and bake according to the fry package directions, using the same temperature and time the manufacturer recommends. Place the baking dish on the center rack so the heat circulates evenly. Keep an eye on it toward the end of the baking time: the fries should be golden and crispy on the edges, and the cheese should be bubbly and fully melted with just a hint of browning in spots. If your oven runs hot or cool, you may need to adjust the baking time slightly, but use the fries’ color and texture as your main guide—they should look similar to how they do when you bake them on a sheet pan.
Once the fries are crisp and the cheese is melted, carefully remove the baking dish from the oven and let it rest for a few minutes. This brief resting time makes it easier to scoop and serve without everything falling apart. Serve it up hot and dig in—enjoy every cheesy, savory bite! You can scoop generous portions with a large spoon or spatula, making sure to get some beef, fries, and cheese in every serving for that full cheeseburger experience in casserole form.
