To remove any unwanted smell, soak the pork in warm water for 10 minutes to drain excess blood. After soaking, pat the pork dry with paper towels.
In a bowl, mix together gochujang, gochugaru, sugar, soy sauce, mirin, sesame oil, minced garlic, and black pepper until well combined.
Add the pork to the bowl and mix well, ensuring every piece is evenly coated. Cover and refrigerate for at least 30 minutes (or overnight for deeper flavor).
Slice the onion into thin strips. Chop the green onion into 2-3 cm pieces.
Mix the marinated pork with the chopped onion and green onion.
In a large pan, heat 1 tablespoon of oil over medium-high heat. Add the mixture to the pan. If using a regular pan, add 50ml of water to prevent sticking (skip this step for a non-stick pan).
Stir-fry for 10-12 minutes, stirring occasionally until the pork is fully cooked. Reduce the heat to medium and cook for another 5 minutes until slightly caramelized. If using thicker cuts, slice the pork into bite-sized pieces before serving.
Plate the Jeyuk-bokkeum and garnish with sesame seeds (optional). Serve hot with steamed rice, lettuce wraps (ssam), or a side of kimchi.