Slice each hot dog lengthwise without cutting all the way through (butterfly style).
Preheat a skillet or griddle over medium heat. Place the hot dogs cut side down and cook for 2 to 3 minutes until browned. Flip and cook for an additional 2 minutes on the other side.
While the hot dogs cook, lightly butter the inside of the buns. Toast the buns cut side down on the griddle until golden brown.
Place a slice of American cheese inside each bun, followed by the grilled hot dog. Layer with a slice of bacon, chopped onions, and any optional toppings you like.
For the authentic Five Guys touch, wrap the hot dogs in foil for 2 to 3 minutes before serving to help the cheese melt and hold everything together.