Melt butter in a large skillet over medium-high heat. Add sliced onion and cook, stirring occasionally, until onion is golden brown and soft. Remove onion from skillet; set aside.
In a large mixing bowl, combine ground beef, salt, pepper and Worcestershire sauce. Form into 6 equal-sized square patties (approximately 5.3 ounces per patty).
Place patties in the preheated skillet, cooking on both sides until you reach desired doneness.
Assemble patty melts starting with one slice of bread, cooked patty, one slice swiss cheese, onion (approximately 19 grams per sandwich), 1⁄2 Tablespoon thousand island dressing and another slice of bread.
Spray a clean griddle or panini press with nonstick cooking spray and cook patty melts until golden brown on each side and cheese is melted. Cut in half and serve.