1) Trim fat from meat. (At this point you can partially freeze meat to make slicing easier.) Thinly slice across the grain into bite-size strips. Set aside.

2) For sauce, drain pineapple, reserving juice. Pour 1/2 cup of the reserved juice into a small bowl. (Reserve remaining juice for another use.) Stir in the vinegar, soy sauce, cornstarch, brown sugar and ground red pepper. Set aside.

3) Add oil to a wok or a large skillet. Preheat over medium-high heat (add more oil if necessary during cooking). Stir-fry green pepper and onion in hot oil for 3 to 4 minutes or until crisp-tender. Remove the vegetables from wok or skillet.

4) Add meat to wok or skillet. Stir-fry for 2 to 3 minutes or to desired doneness. Push meat from center of wok or skillet. Stir sauce; add to center of wok or skillet. Cook and stir until thickened and bubbly.

5) Return cooked vegetables to wok. Add pineapple chunks. Stir all ingredients together to coat. Cook and stir about 2 minutes more or until heated through. Stir in orange sections. If desired, garnish with orange slices.