Combine all of the burger ingredients except the pineapple in a bowl and form into six patties about 1-inch thick. Whisk together sauce ingredients and refrigerate until ready to use. Heat a grill or grill pan to medium and cook the burgers for about 4 minutes on each side or until cooked through. Grill the pineapple rings for 1 minute on each side, then place a ring on top of each burger. Serve with sauce, lettuce, tomatoes, and onions.