Add the bacon to a medium skillet set over medium-low heat. Cook, stirring, until all the fat has rendered and the bacon is crispy. Using a slotted spoon, transfer the bacon to a plate and reserve all drippings.
Brush the outsides of the bread slices with the reserved bacon fat then assemble the sandwich by layering the shredded beef, mozzarella cheese, banana peppers and bacon pieces on one slice of bread. Top with the remaining slice of bread.
Place the sandwich back in the skillet set over medium heat. Cook, undisturbed, until golden brown, then flip once, cover and continue cooking until the cheese is melted. Slice and serve.