Make Horseradish Aioli by mixing the ingredients together in a bowl at least 30 minutes (or up to 2 weeks) before assembling burgers.

Season and form patties: Sprinkle the onion soup mix all over the ground beef and use your hands to gently mix it into the meat (be careful not to over-mix the meat). Divide into 6 equal portions and gently form into ½ inch thick patties that are wider than the burger buns (as they will shrink). Use your thumb to press an indentation into the center of each patty. Cover and set aside.

Make caramelized onions: Melt butter in a large skillet over medium high heat. Add onions to the pan in an even layer and leave them until they begin to brown. Once they start to brown on the bottom, stir in sugar and vinegar. Season with salt and pepper. Cook, stirring occasionally until browned, about 15 minutes.

Grill burgers: Preheat grill to medium high heat. Place patties on hot grill, indent-side up. Close grill lid and cook for 3-4 minutes, until the bottom of the burger is seared and juices are accumulating on top of the burger. Flip and cook an additional 3-4 minutes or until cooked to desired doneness (130-150 degrees).

Add cheese on burgers during the last minute of cooking. Remove to a plate and allow to rest for a few minutes before assembling.

Assemble: Toast burger buns if desired and place a spoonful of aioli on the buns. Top with burger patty, bacon, caramelized onions and arugula or lettuce, then top bun.