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1
Divide the meat into six equal portions.
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2
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 (see Notes).
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3
Set up the grill for direct cooking and preheat to high.
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4
When ready to cook, brush and oil the grill grate.
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5
If using onion slices, brush them on both sides with melted butter and season with salt and pepper.
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6
Place the onion on the hot grate and grill until nicely browned, about 4 minutes per side, then transfer to a plate.
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7
Brush one side of the meat patties lightly with melted butter and season with salt and pepper.
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8
Arrange the burgers, buttered side down, on the hot grate and grill until the bottoms are nicely browned, 4 to 5 minutes.
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9
Brush the tops lightly with some of the melted butter and season them with salt and pepper.
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10
Using a spatula, turn the burgers and grill until they are browned and cooked to taste, 4 to 5 minutes longer for medium.
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11
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.
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12
Set out the toppings.
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13
Put the burgers and onion slices on buns and serve.
