In a medium saute pan over medium heat, cook the bacon until it is crisp and golden brown on both sides, about 4 minutes. Transfer the bacon to a paper towel lined plate and set aside. Remove half of the fat from the pan and set aside.  Add the mushrooms to the pan with the bacon fat and cook over medium heat until the mushrooms begin to brown, about 3 to 4 minutes. Transfer the mushrooms to the plate with the bacon and keep warm. Add the onions to the pan with the remaining bacon fat and cook, stirring often, until the onions are well caramelized about 5 minutes. Add the garlic and continue to cook for another minute. In a small bowl, combine the mushrooms, onions and garlic and season with salt and pepper, to taste.

Divide the mushrooms and onions into 4 piles and lay a piece of cheese over the top. 

Preheat a grill to medium, and lightly oil the grates.

Remove the patties from the refrigerator and place them on the grill. Cook to the desired degree of doneness, 3 to 4 minutes on each side for medium-rare, placing the onion, mushroom and cheese piles on the patties when you flip them over to cook the second side.

Place a burger on the bottom of each of the buns and divide the bacon into 8 pieces placing two pieces on top of the melted cheese. Dress the buns with mayonnaise and mustard if desired and place the top of the buns on each burger.

Serve immediately.