In a large bowl, combine ground beef, salt, pepper, garlic powder and worcestershire sauce. Form mixture into 4 long thin patties.
Melt 2 tablespoons butter in a large skillet over medium-low heat. Add sliced onion and cook for 20-25 minutes, until caramelized. Remove onions from skillet and set aside.
Melt another 2 tablespoons butter over medium heat, add burgers and cook until browned, flip and cook until both sides are browned.
On one side of each slice of bread, place one slice of swiss cheese. On 4 slices of the bread, place one burger patty, and 1/4 of the onion mixture. Top with the remaining slice of bread, cheese side down.
Melt the remaining 2 tablespoons of butter over medium heat. Cook sandwiches in skillet until browned on both sides and cheese is melted.
