Make the sweet bourbon sauce. In a medium saucepan, combine all of the ingredients for the sweet bourbon sauce. Bring to a boil, reduce the heat to medium, and simmer for 30 minutes. Use a blender to puree the sauce before using.
Prep the crispy onion straw batter. In a medium bowl, combine the sliced onions and the buttermilk. In a pie plate gently mix the flour, corn starch, and sweet rub. Place a heavy bottom pot on the stove top and preheat the oil to 400 degrees F.
Preheat the grill. Preheat your grill to high heat (around 450 degrees).
Make the patties. In a large bowl, place your ground beef and pour in the bourbon. Mix very gently. Overmixing can make your burgers tough. Form into 6 individual patties. Season on both sides with the sweet rub.
Grill the burgers. Place the burgers on the grill grate and cook for 6-8 minutes per side, or until burgers are within 10 degrees of desired doneness. The FDA recommends ground beef is cooked to an internal temperature of 160 degrees F.
Make the onion straws. While the burgers cook, fry your onion straws. Remove the onion strips from the buttermilk and toss in the flour/cornstarch mixture. Separate them into individual strips and make sure they are each coated with the flour/cornstarch. Gently transfer them to the hot oil and cook for 4-5 minutes, or until the onion straws are crunchy and golden brown. Remove from the oil using a slotted spoon onto a paper towel and season with salt. Set aside until you are ready to assemble your burgers.
Add the cheese and toast the buns. When the burgers are almost done, slather a bit of the bourbon sauce over each burger. Top each with 2 slices of the Havarti cheese. Butter your buns and place those on the grill grates as well. Close the lid on your grill and cook for 1-2 more minutes, or until the cheese is melted and the buns are toasted.
Assemble the bourbon burgers. To assemble your burgers, place a lettuce leaf on the bottom bun (this prevents the bun from getting soggy) then the cheesy burger, then tomato slices (if desired), a handful of the onion straws, another hearty spoonful of the bourbon sauce, and the buttery toasted top bun. Secure the burger with a large toothpick or skewer. Serve immediately and enjoy!