INGREDIENTS
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2 tablespoons unsalted butter
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2 tablespoons all-purpose flour
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1/4 teaspoon mustard powder
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3/4 cup beer
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1/2 cup half-and-half
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6 ounces grated sharp yellow cheddar cheese
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1 tablespoon horseradish
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1 1/2 pounds ground beef, preferably chuck
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Kosher salt and freshly ground pepper
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Vegetable oil, for the grill
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4 poppy seed rolls, split
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Tomato slices, for topping
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2 tablespoons chopped fresh chives