3 tablespoons Dijon mustard
1 teaspoon white wine vinegar
1/2 teaspoon salt
1/2 teaspoon black pepper
6 (5-ounce) skinless boneless chicken breast halves
1 1/2 English muffins (not sandwich-size)
3/4 cup finely grated Parmigiano-Reggiano (1 1/2 ounces)
1 tablespoon unsalted butter, melted
Preheat oven to 450 degrees and line a baking sheet with parchment.
Whisk together mustard, vinegar, salt, and 1/4 teaspoon pepper in a large bowl and add chicken breasts, tossing to coat well.
Pulse English muffins in a food processor until finely ground. Add cheese, butter, and remaining 1/4 teaspoon pepper and pulse until combined well. Transfer to a shallow soup plate or pie plate.
Dredge chicken, one piece at a time, in crumbs, coating completely and pressing gently to help crumbs adhere, then transfer to baking sheet. Bake in middle of oven until golden brown and cooked through, 15 to 20 minutes.