Cut the pork loin into bite-sized pieces.
Slice the onion and green onion thinly.
In a bowl, mix gochujang, soy sauce, minced garlic, red pepper powder, sugar, and sesame oil to make the marinade.
Add the marinade to the pork and let it marinate.
Heat oil in a pan over medium heat and sauté the onion and green onion.
Add the marinated pork and stir-fry until cooked through.
Taste and adjust seasoning as needed to finish.