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1
In a small bowl, toss the cheese with the cornstarch.
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2
Rub the garlic all over the inside of a medium saucepan, then add the clove to the pan with the lager and bring to a boil.
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3
Add the cheese mixture in large handfuls, stirring until melted, and bring to a boil.
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4
Add the mustard, season with salt and pepper and simmer over moderate heat until thickened, about 3 minutes; keep warm over low heat.
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5
In a medium bowl, lightly knead the sirloin with the chuck and loosely form into 4 patties about 3/4 inch thick.
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6
Season the burgers very generously with salt and pepper and transfer to a plate lined with plastic wrap.
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7
Brush the cut sides of the buns with the melted butter.
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8
Light a grill.
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9
When the fire is medium hot, oil the grate.
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10
Grill the burgers for about 10 minutes, turning once, for medium meat.
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11
Move the burgers away from the heat and grill the cut sides of the buns for about 1 minute, until toasted.
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12
Set a lettuce leaf and a tomato slice on the bottom half of each bun.
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13
Top each with a burger and a generous spoonful of cheese sauce.
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14
Cover with the top half of a bun and serve, passing the remaining cheese sauce on the side.
