Combine buttermilk and mustard in a shallow dish, stirring with a whisk. Add chicken to buttermilk mixture, turning to coat.
Heat a small skillet over medium-high heat. Add panko to pan; cook 3 minutes or until golden, stirring frequently. Combine panko, nuts, and next 4 ingredients (through pepper) in a shallow dish. Remove chicken from buttermilk mixture; discard buttermilk mixture. Dredge chicken in panko mixture.
Arrange a wire rack on a large baking sheet; coat rack with cooking spray. Arrange chicken on rack; coat chicken with cooking spray. Bake at 425° for 13 minutes or until chicken is done. Garnish with rosemary leaves, if desired.
This recipe originally ran in Cooking Light June, 2010 and was updated for the November, 2012 25th anniversary issue.
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