Set your oven to preheat at 375°F. Lightly grease a 9x13-inch rectangular baking pan to prevent sticking.
In a large skillet over medium heat, brown the ground beef with the diced onion until the meat is fully cooked and the onion is tender and translucent. Carefully remove and discard any rendered fat.
Incorporate the steak seasoning into the meat mixture. Stir in the ketchup, mustard, Worcestershire sauce, cheddar cheese soup, and mushroom soup. Allow this combination to simmer gently for approximately 5 minutes to meld the flavors.
Transfer the beef and sauce mixture to the prepared baking dish, smoothing it into an even layer.
Scatter the shredded cheddar evenly across the top of the meat layer.
Arrange the frozen fries over the cheese in a single, uniform layer to ensure even crisping.
Bake uncovered for 40 to 45 minutes, or until the fries are crisped and golden on top.
Let the casserole rest for a few minutes once removed from the oven before slicing and serving.
Optional garnishes include chopped pickles, shredded lettuce, diced tomatoes, or additional condiments like ketchup and mustard.