In a small bowl, mix together the orange juice, lemon juice, and olive oil. Place the chicken breasts in a shallow container and season generously with salt and pepper. Pour the juice mixture over the chicken.
Cook the chicken on a grill or in a skillet over medium heat. Cook for about 5 minutes on each side, or until the internal temperature reaches 165 degrees in the middle of the chicken. Keep a close eye on the temperature so the chicken doesn’t come out dry.
Let the chicken cool a bit, then cut it into thin strips. Set aside.
Place the lettuce in a large bowl and dress it with the citrus vinaigrette. Transfer the lettuce to a serving platter. Top with the chicken, red pepper, mandarins, grapefruit, and walnuts. Enjoy!