Blue Cheese, Bacon, and Balsamic Onion Burger

In a small bowl, toss together the onion, vinegar, and 1/4 teaspoon each salt and pepper. Let sit, tossing occasionally, until ready to use. Heat grill to medium-high. Gently form the beef into 4 balls. (Don't overwork the meat — this can result in a tough, dry burger.) Flatten each ball into a 3/4-inch- thick patty. Using your thumb, make a shallow 1 1/2-inch-wide indent in the top of each patty. Season the patties with 1/2 teaspoon each salt and pepper. Place the patties on the grill, indent facing up, and cook until the burgers release easily from the grill, 3 to 4 minutes. Flip and cook to desired doneness, 3 to 4 minutes more for medium. During the last 2 minutes, top with the blue cheese and grill, covered, until gooey. If desired, grill the buns until lightly toasted. Serve the burgers on buns and top with the balsamic onions, lettuce, tomato, and bacon, if desired.