In a large skillet, cook the bacon over medium heat until crispy. Remove the bacon from the pan and reserve for another use. Allow the bacon fat to cool in the pan then transfer to a small bowl.
Whisk the bacon fat, mustard, honey and horseradish together until smooth. Cover and refrigerate until ready to use. (Sauce can be made up to 24 hours in advance.)
BURGERS:
Using clean hands, blend together the ground beef, salt and pepper to combine.
Divide the beef into four equal portions and shape into patties approximately 4 to 5 inches in diameter. Set aside.
In a small saucepan over medium heat, whisk together the maple syrup and bourbon. Bring the mixture to a boil and cook for 4 to 5 minutes until it reduces by half. Remove from the heat and set aside.
Cook the burgers on the grill over high heat for 2 to 3 minutes. Turn the burgers over and cook for another 1 to 2 minutes. Brush the maple glaze over the burgers and turn the heat down.
Top each burger with one slice of cheese and cook for another minute or until the cheese is melted.
Transfer to a platter and rest for 2 to 3 minutes.
Arrange the burgers on the buns and top with the mustard sauce, tomato, lettuce and onion, if desired. Serve immediately.