Cut the pork sirloin into bite-size pieces.
Slice the onion, green onion, and garlic.
Make the marinade (gochujang, soy sauce, chili powder, sugar, minced garlic, sesame oil, pepper).
Add the marinade to the pork and vegetables, and gently mix and coat.
Heat oil in a pan over medium heat and stir-fry the pork and vegetables.
Continue to stir-fry until the meat is cooked.
Add sesame oil and finish.