Whisk together the gochujang, gochugaru, soy sauce, sugar, rice wine, grated onion, garlic, ginger and black pepper.
Toss the pork in the marinade and let it sit at least 30 minutes at room temperature, or up to overnight refrigerated for a deeper flavor.
Heat the oil in a wide skillet or wok over high heat until it's shimmering.
Add the pork in a single layer, in batches if your pan isn't wide enough to avoid crowding, and let it sear undisturbed for 1 to 2 minutes before stirring.
Cook, stirring occasionally, until the pork is mostly cooked through and the sauce has started to thicken and cling, about 5 to 6 minutes.
Add the onion and cabbage. Stir-fry over high heat for 2 to 3 minutes, just until they've softened at the edges but still have some bite.
Add the fresh chili in the last minute, if using.
Take the pan off the heat, stir in sesame oil, and scatter scallions and sesame seeds over the top.