Heat 1 tbsp butter in a skillet and cook sliced onions over medium heat 10–12 minutes until caramelized, then set aside.
Season ground beef with salt, pepper, and garlic powder and form into 4 thin oval patties.
Cook patties in skillet 3–4 minutes per side until cooked through, then remove.
Spread butter or mayonnaise on one side of each bread slice.
Place bread butter-side down in skillet, add Swiss cheese, beef patty, caramelized onions, more cheese, and top with another bread slice.
Cook 2–3 minutes per side until bread turns golden and cheese melts fully.
Slice in half and serve immediately while hot and crispy.