01 - In a large skillet over medium heat, melt 2 tablespoons of butter. Add the sliced onions and cook, stirring frequently, until deeply caramelized and golden brown, about 12 to 15 minutes. Remove the onions from the skillet and set aside.
02 - While the onions cook, season the ground beef with salt, black pepper, and garlic powder. Form the mixture into 4 thin, wide patties that match the size and shape of your bread slices.
03 - In the same skillet over medium-high heat, cook beef patties for 3 to 4 minutes per side, or until the patties are browned and cooked through. Remove and set aside.
04 - Place 8 bread slices on a surface and spread softened butter on one side of each slice. On the unbuttered side of 4 slices, add a slice of cheese, one beef patty, some caramelized onions, and another slice of cheese. Top with remaining bread slices, buttered side facing out.
05 - Preheat a clean skillet or griddle over medium heat. Place each assembled sandwich on the skillet, pressing gently. Cook for 2 to 4 minutes per side, or until the bread is golden and crisp and the cheese is fully melted.
06 - Remove sandwiches from the skillet and slice each in half. Serve immediately while hot.