Cut pork into thin slices (⅛”). If slightly frozen, it's easier to cut.
Peel and slice a medium onion. Wash and cut green onions into 2 inches. Mince garlic and ginger. Wash and slice green chili peppers (optional).
Mix gochujang, sugar, minced garlic, minced ginger, chopped green onions, black pepper, sesame oil, sliced onions, chopped green chili peppers, sesame seeds, soy sauce, and cheongju.
Place the meat in a bowl, add the marinade, and mix thoroughly. Cover and refrigerate for at least an hour.
Coat a frying pan with vegetable oil and cook the marinated meat on high heat for 10-15 minutes until fully cooked and browned.
Serve on a plate and sprinkle sesame seeds to garnish. Enjoy with rice or in lettuce wraps.