- Cut beef across grain into 1/2-inch thick strips, then into 1-inch squares.
- Combine teriyaki sauce and sugar in measuring cup, stirring until sugar dissolves. Remove 3 Tbsp. of mixture and pour over beef in large plastic food storage bag. Press air out of bag; close top securely. Turn bag over several times to coat pieces well. Marinate 15 minutes.
- Meanwhile, add enough water to remaining teriyaki sauce mixture to measure 2/3 cup; blend in cornstarch.
- Heat oil in large skillet over high heat; add beef and sauté 1 minute. Add teriyaki sauce mixture; cook, stirring, about 1 minute, or until sauce boils and thickens slightly.
- Spoon beef and sauce over rice in large individual serving bowls. Serve vegetables on top of rice or on the side, as desired.