Prepare the pork by trimming if necessary.

Make the marinade by mixing gochujang, red pepper flakes, soy sauce, corn syrup, sugar, sesame oil, black pepper, and minced garlic in a bowl.

Mix the pork with the marinade thoroughly using gloves. Optionally, let it marinate overnight for enhanced flavor.

Heat a pan or skillet over medium heat and add the marinated pork. Cover and cook until halfway done.

Add sliced onions and continue stir-frying until the pork is fully cooked and onions are tender.