Trim excess fat from the pork shoulder. Using a sharp knife, slice the pork very thinly across the grain into roughly 1/8–1/4-inch slices (freeze 20–30 minutes first if the meat is too soft to slice thinly).

Combine marinade

In a medium bowl combine soy sauce, rice wine (or dry sherry), sesame oil, brown sugar, grated ginger, minced garlic, cornstarch and black pepper. Whisk until sugar and cornstarch are dissolved and mixture is uniform.

Add the sliced pork to the marinade and toss to coat evenly. Cover and refrigerate for at least 20 minutes and up to 2 hours. The cornstarch will help the marinade cling and slightly thicken during cooking.

Cook and Finish

Prepare vegetables

Thinly slice the onion and slice the green onions, keeping white and green parts separate if preferred.

Heat pan

Heat a large skillet or wok over medium-high heat until hot. Add vegetable oil and swirl to coat.

Sear the pork: Remove pork from refrigerator and let excess marinade drip off (reserve marinade). Working in a single layer (in two batches if necessary), add pork slices to the hot pan in an even layer. Let them sear without stirring for 30–40 seconds to develop color, then stir and continue to cook until pork is cooked through and lightly browned, about 2–3 minutes total per batch. Transfer cooked pork to a plate and keep warm.

Cook the onion: In the same hot pan add the sliced onion and cook 1–2 minutes, scraping up any brown bits. If pan looks dry, add 1 tablespoon water or stock to deglaze.

Return pork and finish sauce: Return all pork to the pan along with any accumulated juices and the reserved marinade. Add remaining 1 tablespoon water (or stock) if needed. Cook and stir 1–2 minutes until sauce thickens slightly and pork is glazed. Taste and season with salt (about 1/4 tsp) or additional soy sauce if desired.

Garnish and serve: Remove from heat. Sprinkle sliced green onions and toasted sesame seeds over the pork. Serve immediately over steamed rice or noodles.