1. Stack beef steaks; cut lengthwise in half, then crosswise into 1-inch wide strips. Combine marinade ingredients in medium bowl. Remove and reserve 2 tablespoons mixture. Add beef to remaining marinade; toss to coat. Cover and marinate in refrigerator 30 minutes to 2 hours.
2. Combine vegetables and 1/4 cup water in large nonstick skillet; cook over medium-high heat 4 to 5 minutes or until most of water is evaporated and vegetables are hot, stirring occasionally. Remove from skillet and set aside.
3. Heat same skillet over medium-high heat until hot. Add 1/2 of beef; stir-fry 1 minute or until outside surface of beef is no longer pink. (Do not overcook.) Remove from skillet. Repeat with remaining beef. Combine cornstarch mixture and reserved marinade mixture. Return beef and vegetables to pan. Add cornstarch mixture; cook and stir 1 minute or until thickened and bubbly. Serve over rice. Sprinkle with walnuts, if desired.