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1. Set up the grill for direct grilling and preheat to high.
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2. Place the chuck and sirloin in a large mixing bowl and mix with a wooden spoon, or mix the meat in a stand mixer fitted with a dough hook. If possible, avoid mixing the meat with your hands so your fingers don't warm it.
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3. Wet your hands with cold water and divide the meat into 8 equal portions. Working quickly and with a light touch, pat each portion into a 1/2-inch-thick squarish patty. Generously season each patty on both sides with salt and pepper. Press an onion wedge into one side of each patty so that it's flush with the meat.
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4. Lightly brush the bread slices with the butter, if using. Arrange the tomatoes and lettuce leaves on an attractive serving platter.
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5. When ready to cook, brush and oil the grill grate. Place the burgers on the hot grate, onion side down. Grill the burgers until cooked to taste, 3 to 4 minutes per side for medium-rare. To test for doneness, insert an instant-read meat thermometer through the side of a burger into the center. The internal temperature should be about 145u00b0F for medium-rare or, if using commercial ground beef, cook it to at least medium, 160u00b0F.
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6. Meanwhile, place the bread slices on the hot grate and grill until lightly toasted, 1 to 2 minutes per side.
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7. To serve, place a lettuce leaf on top of a slice of toast. Top with a burger, tomato slice, and
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, if using. Slap a piece of toast on top and serve at once.
