Cut the pork into bite-size pieces.
Mix gochujang (Korean chili paste), soy sauce, minced garlic, sugar, red pepper flakes, and sesame oil to make the marinade.
Coat the pork with the marinade and let it sit for 10 minutes.
Heat cooking oil in a pan over medium heat and stir-fry the pork.
Add onion and green onion, and stir-fry together.
Once the meat is cooked through, plate and serve.