Cut onion in half, then into thin slices. Finely chop some of the slices to equal 1/4 cup chopped onion; set the slices aside. Heavily coat a small nonstick pan with nonstick spray; heat over medium heat, then add chopped onion and 1 teaspoon of the cinnamon chipotle rub. Cook 1 minute or until fragrant. Combine seasoned onion with ground beef in a large bowl. Shape into 16 small burgers, each about 1-inch thick. Can be made ahead and covered with plastic wrap; refrigerate for up to 6 hours.

Heat oil in a large nonstick skillet over medium heat. Add reserved onion slices and cook, stirring often, 10 minutes or until lightly golden. Sprinkle remaining 3 teaspoons cinnamon-chipotle rub and cook 1 minute. Add bourbon and cook 2 minutes, until liquid is reduced. Stir in ketchup and mustard; simmer 2 minutes. Can be made ahead; cover and refrigerate for several hours.

Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. Grill burgers about 6 minutes per side or until instant-read thermometer registers 160 degree F. Reheat onions if necessary. Place patties on buns, topping each burger with a tablespoon of onions; serve remaining onions on the side.

Note: If you cannot find the rub, stir together 1 teaspoon ground cinnamon and 4 teaspoons chipotle or chili powder (you'll have a little extra).