In a small bowl, stir together the mayonnaise, garlic, smoked paprika, and salt. Set aside.
Make the burgers
Generously butter the insides of the hamburger buns and set aside.
In a large bowl, combine the ground beef, sausage, salt, onion powder, and garlic powder. Mix until all the ingredients are well incorporated. Form the meat into 16 patties, about 4 1/2 inches in diameter and and 1/4 inch thick. You want the patties to be thin, because each burger will get two. Separate each patty with a piece of wax paper so they don’t stick together, and can easily be transferred to the grill.
Preheat the grill to medium-high heat.
Brush the onion rounds with the olive oil, and season with salt and black pepper.
Place the patties and onion rounds directly on the hot grill. Cook the onions for 3 minutes on each side, until they are softened and have nice char lines. Cook the patties for 2 to 3 minutes and flip with a spatula. Place a slice of cheese on top of each patty, so half of the patties are topped with cheddar and half with Havarti. Cook for 2-3 minutes longer, until the patties are no longer pink on the outside and the cheese has melted.
Transfer the onion slices and burgers to a serving platter or large baking sheet. Cover with foil to keep warm.
Place the buttered hamburger buns cut side down on the grill and grill until toasted, 30-60 seconds.
To assemble the burgers, spread the aioli on the insides of the toasted buns. Top the bottoms with two patties, and cheese. Add the grilled onions, lettuce, tomato slices (pickles are optional), and bacon and tops. Serve immediately.