Grate 2 teaspoons orange peel.  Halve 1 orange, squeeze to yield 1/2 cup juice.  (If orange doesn’t have enough juice, use a second one as needed.) Peel and slice remaining oranges.  Set aside. 

Flatten chicken with mallet; use waxed paper to prevent splatter.  Sprinkle with pepper (and salt if desired).

Sauté chicken in 2 teaspoons oil in skillet; turning once.  Cook until done but still moist and tender.  Transfer chicken to a serving dish. 

To prepare the sauce; add orange peel, juice, onion and mustard to pan and cook, stirring.  Add orange slices and pistachios and heat thoroughly.  Pour over chicken.