INGREDIENTS
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2 Tablespoons unsalted butter
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1 small onion, thinly sliced
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8 ounces beer
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1/4 cup beer mustard, or more if needed (you can buy store-bought or make it homemade, but I just mixed a few tablespoons of Dijon mustard with a little bit of beer)
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1 1/2 pounds ground beef
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burger seasonings (seasoned salt, pepper, etc.), if preferred
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8 to 12 thin slices black forest ham
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4 slices Swiss cheese
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4 lettuce leaves
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4 pretzel rolls