Cut the pork into bite-sized pieces.
Julienne the onion, green onion, and chili pepper.
In a bowl, mix together soy sauce, sugar, minced garlic, black pepper, and sesame oil to make the marinade.
Add the marinade to the pork and let it marinate for 10 minutes.
Heat cooking oil in a pan and sauté the green onion to make green onion oil.
Add the marinated pork to the pan and stir-fry.
Add the onion and chili pepper, then stir-fry everything together.
Once the meat is cooked through, finish with a drizzle of sesame oil.