INGREDIENTS
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3 tablespoons Dijon mustard
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1 teaspoon white-wine vinegar
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1/2 teaspoon salt
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1/2 teaspoon black pepper
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6 (5-oz) skinless boneless chicken breast halves
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1 1/2 English muffins (not sandwich-size)
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3/4 cup finely grated Parmigiano-Reggiano (1 1/2 oz)
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1 tablespoon unsalted butter, melted