Melt butter in a skillet over medium heat. Add the sliced onions. Give them a good stir to coat in the butter. Allow the onions to cook for 10 minutes without stirring. With a spatula, flip the onions over so the bottoms is now on top. Allow to cook for an additional 10 minutes without stirring. Lower the heat to medium-low and stir the onions. Allow to cook an additional 10-15 minutes or until the onions are caramelized. Remove from the heat and set aside.

Preheat a pan to high heat. In a medium bowl, combine the ground beef with the salt and pepper. Handle gently, mixing the seasoning into the meat. Divide and shape the meat into 4 patties. Place burgers in the pan and cook until the bottom is browned and easily releases from the pan. Carefully flip the patties and continue to cook to desired doneness. Transfer to a plate to rest.

Lay the bread out on wax paper. Butter one side of each slice of bread. Flip 4 slices of bread over and top with the desired amount of Thousand Island salad dressing. Add a patty to the top of the dressing. Divide the onions between the 4 sandwiches. Top each sandwich with a slice of cheese and the remaining bread slices, butter side up.

Place sandwiches in a preheated non-stick pan or griddle. Cook until the bottom slice of bread is browned. Carefully flip the sandwich over and gently press down, cooking until the other side until is browned. Remove the sandwich to a serving plate and slice it in half. Serve with additional Thousand Island if desired.