Sauté onions in oil or bacon fat for 3–4 minutes. Add salt, pepper, sugar, and butter. Cook for 15–20 minutes until golden, then stir in bourbon and cook 1 more minute. Set aside.
Heat grill or grill pan to 375–400°F.
Mix ground beef, bourbon, Worcestershire, garlic powder, salt, and pepper. Shape into 6 patties, pressing a dent in the center of each.
Grill 3 minutes per side with the lid closed. Use a thermometer to check for 120°F (it will rise to 130°F after resting).
Reduce heat, top patties with cheese, and grill 1 more minute to melt.
Let burgers rest on a tray for 3 minutes.
Toast buns. Stack each with a patty, 2 bacon slices, caramelized onions, and the top bun. Serve warm.