1 Cut the chicken into 1 -inch cubes. Place the chicken in a bowl and add 1 tablespoon of the oil, 1 tablespoon of the soy sauce and 1 tablespoon of the cornstarch. Toss to thoroughly coat the chicken.
2 Cut the bell pepper into 1-inch squares. Halve the onion and cut it into 1/4 inch-thick slices. Separate the broccoli florets from the stem and cut the stem into Winch-thick slices.
3 In a small bowl, combine the broth, sugar, red pepper flakes and the remaining 1 tablespoon soy sauce and 1 1/2 teaspoons cornstarch.
4 In a large skillet or wok, warm 1 tablespoon of the oil over medium-high heat. When the oil is very hot but not smoking, add the chicken and the marinade and stir-fry for 2 to 3 minutes, or until the chicken is opaque but still slightly pink in the center. Transfer the chicken to a plate and set aside.
5 Add the remaining 1 tablespoon oil to the skillet. Add the bell pepper, onion, broccoli, garlic and water chestnuts and stir-fry until the onions begin to wilt, 2 to 3 minutes.
6 Return the chicken to the skillet and add the walnuts. Stir the broth mixture, add it to the skillet and bring the liquid to a boil.
7 Cook, stirring constantly, until the vegetables are crisp-tender and the chicken is cooked through, 2 to 3 minutes longer.