Place a cast iron skillet on the grill. Heat olive oil in skillet. Add onion slices. Sauté for about 10 minutes. Add the whiskey, and season with salt and pepper to taste. Reduce the heat to low, or move the pan to indirect heat. Continue sautéing for about 5 minutes, until caramelized. Set aside.
Heat the grill to high.
Shape the beef into 5 patties. Sprinkle liberally with salt and pepper. Using your thumb, press a dimple into the center of the burger.
Place the burgers on the hot grill. Grill about 5 minutes. Flip. Grill 5 more minutes. Add a slice of cheese on top until melted. Remove to a platter and cover with foil to keep warm.
Place the buns on the grill cut side down for 15-30 seconds, until toasted.
Spread mayonnaise on the bottom bun. Layer in this order: shredded lettuce, tomato slice, cheeseburger, caramelized onions, beef bacon and the top bun.