What makes a great hamburger? First there’s the meat. You want to use a flavorful cut, like sirloin (for uptown burgers), or chuck or round (if you’re feeling more democratic). And it shouldn’t be too lean: 15 to 20 percent fat is ideal.
Step 1: Divide the meat into six equal portions. Lightly wet your hands with cold water, then form each portion of meat into a round patty, 4 inches across and of an even thickness.
Step 2: Set up the grill for direct cooking and preheat to high.
Step 3: When ready to cook, brush and oil the grill grate.
Step 4: If using onion slices, brush them on both sides with melted butter and season with salt and pepper. Place the onion on the hot grate and grill until nicely browned, about 4 minutes per side, then transfer to a plate.
Step 5: Brush one side of the meat patties lightly with melted butter and season with salt and pepper. Arrange the burgers, buttered side down, on the hot grate and grill until the bottoms are nicely browned, 4 to 5 minutes. Brush the tops lightly with some of the melted butter and season them with salt and pepper. Using a spatula, turn the burgers and grill until they are browned and cooked to taste, 4 to 5 minutes longer for medium. Meanwhile, brush the cut sides of the buns with the remaining melted butter and toast them, cut sides down, on the grill during the last 2 minutes the burgers cook.
Step 6: Set out the toppings. Put the burgers and onion slices on buns and serve.
If you are not planning on cooking the burgers immediately, place them on a large plate, cover them loosely with plastic wrap, and refrigerate them.