Dwaeji bulgogi marinates so fast because the pork is sliced paper-thin - it absorbs all that gochujang and gochugaru instantly so you can cook it right away. The sugar in the marinade burns easily, so you need to keep things moving in the pan, but that char is exactly what you want.
Mix sesame oil, gochugaru, soy sauce, mirin, gochujang, brown sugar, garlic, ginger, and black pepper in a large bowl.
Add sliced pork shoulder and onions to marinade, coating thoroughly.
Heat neutral oil in large non-stick pan over medium-high heat.
Add marinated pork in even layer, cook undisturbed 2-3 minutes until charred.
Flip and stir frequently until pork is cooked through and glazed, about 5-7 minutes more.
Top with sesame seeds and chopped green onions.
Notes
Serve with rice or as lettuce wraps with red leaf lettuce, perilla leaves, jalapenos and ssamjang. Watch heat level as marinade can burn due to sugar content.