Lightly shape beef into six ½-inch thick patties. Brush onion slices with 1-1/2 teaspoons melted butter, if desired.
Place onion slices on grid over medium ash-covered coals. Grill, covered, 15 to 20 minutes (over medium heat on preheated gas grill, covered, 15 to 20 minutes) or until tender. Place patties on grid over medium, ash-covered coals. Grill, uncovered, 11 to 13 minutes (over medium heat on preheated gas grill, covered, 7 to 8 minutes), turning occasionally, until instant-read thermometer inserted horizontally into center registers 160 F. Season with salt and pepper, as desired.
Brush cut sides of buns with remaining butter, if desired. Place cut sides down on grid during last minute of grilling.
Serve burgers on buns and top with toppings, if desired. Close sandwiches.
Notes
If using 95 percent lean ground beef, add ¼ cup soft bread crumbs and 1 egg white to ground beef in step 1.
Recipe by Webicurean at http://blog.webicurean.com/2008/06/05/the-great-american-hamburger/