Slice the pork as thin as you can against the grain. Putting it in the freezer for 20 minutes first firms it up and makes thin slicing much easier.
In a bowl, stir together the gochujang, gochugaru, soy sauce, brown sugar, mirin, sesame oil, garlic, and ginger. Add the pork and toss to coat. Let it sit 15 to 30 minutes if you have time, or cook right away.
Heat the neutral oil in a wok or large skillet over high heat until it shimmers. Add the onion and stir for 1 minute to soften the edges.
Add the pork and its sauce in as flat a layer as you can. Let it sear undisturbed for about 1 minute, then stir-fry for 4 to 5 minutes until the pork is cooked through and the sauce turns thick and glossy.
Toss in the green onions and cook 30 seconds more. Scatter sesame seeds on top and serve hot over rice or with lettuce for wrapping.