1. Preheat oven to 450. Put a sheet of parchment paper on a rimmed baking sheet.
2. Whisk together the mustard, white wine vinegar, salt and 1/4 tsp pepper in a medium bowl. Add chicken and toss to combine. Set aside (you can cover and refrigerate for up to 2 hours if you would like).
3. Pulse the English muffins in a food processor until finely ground. Add the cheese, butter, remaining 1/4 tsp pepper and pulse until well combined. Transfer to a shallow dish or pie plate.
4. Dredge chicken one piece at a time in the crumbs, pressing gently so they adhere. Then transfer to the baking sheet.
5. Bake until golden brown and cooked through, 20 minutes.
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