Cut the beef into bite-sized pieces.
Slice the onion, green onion, and garlic.
In a bowl, mix gochujang (Korean red chili paste), red pepper powder, soy sauce, minced garlic, sugar, and sesame oil to make the marinade.
Marinate the sliced beef in the sauce for 10 minutes.
Heat cooking oil in a pan and stir-fry the onion and green onion.
Add the marinated beef and stir-fry over high heat.
Once the beef is cooked, add spicy chili peppers and stir-fry together.
Check the seasoning and finish.