Preheat the oven to 425 degrees Fahrenheit. Prepare the baking sheet pan by placing a wire cooling rack inside of the pan.
Pound the chicken using a mallet to about ½-inch in thickness. This is optional but will help the chicken to cook faster and more evenly.
Combine the buttermilk and mustard in a shallow dish, stirring with a whisk. Add the chicken to the buttermilk turning to coat.
In a small skillet over medium heat, toast the panko breadcrumbs until golden brown, about 3 minutes. Remove the breadcrumbs from the heat. Combine the breadcrumbs, walnuts, parmesan cheese, rosemary, salt, and pepper in a shallow bowl.
Remove that chicken from the buttermilk mixture and dredge each piece in the crumb mixture.
Arrange the chicken on the wire rack of the prepared baking sheet pan. Alternatively, you can line the pan with aluminum foil and spray it with cooking spray.
Bake for 13-15 minutes or until the chicken is no longer pink. Garnish with rosemary leaves.