Parmesan Chicken

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3 tablespoons Dijon mustard

1 teaspoon white wine vinegar

1/2 teaspoon salt

1/2 teaspoon freshly ground black pepper

6 small skinless, boneless chicken breast halves (scant 2 pounds total), rinsed and patted dry

1 1/2 English muffins

3/4 cup finely grated Parmigiano-Reggiano

1 tablespoon unsalted butter, melted

Special equipment: parchment paper