Melt the 4 Tbsp butter in a large skillet, then stir in the white sugar until it dissolves into the butter. Add the sliced or chopped onion pieces into the butter and let simmer over low heat for 30 minutes, stirring often, allowing them to caramelize.
Brown the ground beef with the garlic salt, and drain of any fat.
Preheat the oven to 350 F. Lightly grease a 9x13-inch baking dish with non-stick cooking spray.
Cut the sweet rolls in half and place the bottoms into the prepared baking dish. Spoon the ground beef onto the bottoms of the rolls, then top with the caramelized onions, and slices of Gouda or Swiss cheese. Place the tops of the rolls on top.
In a small mixing bowl, melt the butter and brush over the tops of the rolls.
Bake in the preheated oven for 12 to 15 minutes, or until cheese has melted. Pull apart and serve. Enjoy!