1 tablespoon Dijon mustard
Preheat oven to 450°F. Combine 1 teaspoon oil, five-spice powder, 1/2 teaspoon salt and 1/2 teaspoon pepper in a small bowl. Rub the mixture into both sides of the chicken breasts.
Heat 1 teaspoon oil in a large ovenproof nonstick skillet over medium-high heat. Add chicken breasts; cook until browned on one side, 3 to 5 minutes. Turn them over and transfer the pan to the oven. Roast until the chicken is just cooked through (an instant-read thermometer inserted into the center should read 165°F), 6 to 8 minutes. Transfer the chicken to a cutting board; let rest for 5 minutes.
Meanwhile, peel and segment two of the oranges (see Tip), collecting segments and any juice in a large bowl. (Discard membranes, pith and skin.) Add the greens, bell pepper and onion to the bowl. Zest and juice the remaining orange. Place the zest and juice in a small bowl; whisk in vinegar, mustard, the remaining 4 teaspoons oil, remaining 1/2 teaspoon salt and freshly ground pepper to taste. Pour the dressing over the salad; toss to combine. Slice the chicken and serve on the salad.