In wok, heat sesame oil over high heat. Stir-fry garlic, green onions for 3-5 minutes, until vegetables are crisp-tender.
Add pork and stir-fry for 5 minutes. Add pineapple juice; heat thoroughly.
In small bowl stir together cornstarch, vinegar, soy sauce and brown sugar; stir this mixture into wok and cook and stir until mixture comes to a boil.