Preheat oven to 425. Pound the chicken so that they are about 1/2 thickness. This is optional but will help the chicken to cook faster.
Combine 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 crumbs until golden brown about 3 minutes. Remove from heat and combine the panko crumbs with finely chopped walnuts, parmesan cheese, rosemary, salt and pepper.
Remove chicken from buttermilk mixture and dredge in crumb mixture.
You can either arrange a wire rack on a large baking sheet, or line the pan with aluminum foil and spray with cooking spray. Lay the chicken on top.
Bake at 425 for 13-15 minutes or until chicken is no longer pink. Garnish with Rosemary leaves.