1. Form the ground beef into 4 patties. Season with salt and pepper.
2. Heat a skillet over medium heat. Cook the patties for about 4-5 minutes per side, or until cooked to your liking. Remove and set aside.
3. In the same skillet, add a bit of butter and cook the sliced onions until caramelized. Remove and set aside.
4. Butter one side of each bread slice. Place 4 slices, butter-side down, on the skillet. Top each with a slice of cheese, a beef patty, onions, another slice of cheese, and another slice of bread, butter-side up.
5. Cook until the bread is golden brown and the cheese is melted, about 3-4 minutes per side. Serve hot.