1. Preheat oven to 425°F. Place a wire rack on a foil-lined baking sheet and spray with cooking spray. Set aside.

Make Rosemary Walnut Crust:

1. In a small skillet over low heat, toast panko breadcrumbs until golden, stirring frequently. Remove from heat and allow to cool slightly.
2. Place panko breadcrumbs and walnuts in the bowl of a food processor. Pulse 5-10 times, until finely chopped. (You can skip this step if you can chop your walnuts small enough, but I found the finer consistency from a brief whirl in the food processor made for easier coating.)
3. Combine panko, walnuts, Parmigiano-Reggiano, rosemary, salt and pepper in a shallow bowl. (If making a large batch to freeze extra, portion out some for immediate use and store extra in an airtight container or ziplock bag.)

Assemble and Bake the Chicken:

1. In a shallow bowl, whisk together buttermilk and mustard until combined. Add chicken to mixture, turning to coat. Dredge chicken through panko-walnut mixture, ensuring that both sides are evenly coated.
2. Place chicken on prepared baking sheet/wire rack. Spray top of chicken with cooking spray. Bake for approximately 13 minutes (less time for thinner slices, more for thicker), or until chicken is cooked through.
3. Serve hot.