Partially freeze pork. Cut across the grain into 2 1/2 x 2 1/4-inch strips; set aside. Preheat a wok or large skillet over high heat; add oil. Stir-fry green pepper and onion in hot oil for 2-3 minutes or till crisp-tender. Remove from wok. Add more oil, if necessary. Add half the pork to wok; stir-fry until browned. Remove pork; stir-fry the remaining pork. Return all pork to wok; keep warm.
Drain pineapple, reserving juice. In a small saucepan combine the juice, brown sugar, vinegar, cornstarch and soy sauce. Bring to a boil; cook about 1 minute or till thickened, stirring constantly.
Return green pepper and onion to wok. Stir in pineapple and the thickened pineapple juice mixture. Cook and stir until heated through. Serve with rice, if desired.
Serves 4