Preheat the oven to 400 degrees F (200 degrees C). Grease a baking dish with butter.
Cook ground beef and onion in a large, nonstick skillet over medium heat until beef is browned and onions are soft and translucent, 5 to 10 minutes. Add garlic and cook until fragrant, about 1 minute. Season with salt and pepper. Stir in 1/2 cup Cheddar cheese until well combined.
Remove from heat. Add ketchup, mayonnaise, pickles, and mustard. Transfer mixture to buttered baking dish and top with remaining 1/2 cup Cheddar cheese.
Bake in the preheated oven until cheese is melted, 7 to 10 minutes.
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