1. Preheat oven to 425°. Line a baking sheet with parchment paper.
2. Place pistachios in a shallow bowl. Whisk together mustard, orange juice, salt, and 1/4 teaspoon pepper in a second shallow bowl. Coat 1 side of each chicken breast with mustard mixture, and dip coated side of chicken in chopped nuts. Place chicken, nut sides up, on prepared baking sheet. Sprinkle with remaining 1/4 teaspoon pepper.
3. Bake 20 minutes or until chicken is cooked through and nuts are lightly browned. Sprinkle chicken with orange zest, if desired. Serve with Citrus-Herb Roasted Baby Carrots.