Toss the bacon and diced onions on the griddle. Cook until the bacon gets crisp and the onions are golden brown. Slide the bacon/onion mixture to the side of the griddle.
Cook until golden brown and toasted then flip and toast the other side for about 1-2 minutes until golden brown. Place the rolls on a tray or plate and cover them with a dish towel to keep them warm.
Melt the remaining 2 Tbsp of butter on the griddle. Place the balls of ground beef on the griddle and smash them flat with a large spatula. They should be a bit larger than the rolls because they will shrink as they cook.
Immediately place the cheese slices on the burger patties. Cover the patties with a basting lid to melt the cheese. If you don’t have a basting lid, you can use a metal lasagna pan.