Pat the pork shoulder dry with paper towels. Trim any large pieces of excess fat. Slice the pork across the grain into 1/4-inch thick slices (or thin strips) to maximize tenderness and absorption of the marinade.

Make Marinade

In a medium bowl combine soy sauce, brown sugar, rice vinegar, sesame oil, minced garlic, grated ginger, sliced scallions (reserve a few green parts for garnish), black pepper, and red pepper flakes if using. Stir until the sugar is mostly dissolved.

Marinate Pork

Place the sliced pork in a resealable plastic bag or shallow dish. Pour the marinade over the pork, pressing out excess air if using a bag, and massage so each piece is coated. Refrigerate and marinate for at least 30 minutes and up to 4 hours (longer for deeper flavor, but not more than 8 hours with this cut).

Cooking & Finishing

Heat Pan

When ready to cook, remove the pork from the refrigerator and let sit at room temperature for 10 minutes. Heat a large heavy skillet or cast-iron pan over medium-high heat. Add the vegetable oil and swirl to coat.

Sear Pork

Remove the pork from the marinade, letting excess drip back into the bowl. Reserve the marinade. Arrange pork slices in a single layer in the hot pan, working in batches if necessary to avoid overcrowding. Sear 2 to 3 minutes per side until a golden-brown crust forms and internal temperature reaches 145°F (63°C) for slices this thin. Transfer cooked pork to a plate and tent loosely with foil.

Make Pan Sauce

Lower the heat to medium and carefully pour the reserved marinade into the pan along with 1/2 cup chicken broth or water, scraping any browned bits from the bottom with a wooden spoon. Bring to a simmer and cook for 2 minutes to reduce and to eliminate any raw soy-garlic taste.

Thicken Sauce

Stir the cornstarch into 1 tablespoon cold water to make a slurry. Slowly whisk the slurry into the simmering sauce until it thickens and becomes glossy, about 30–60 seconds. Taste and adjust seasoning as needed (a pinch of salt or a little additional sugar if you prefer sweeter).

Combine and Finish

Return the seared pork and any accumulated juices to the pan and toss in the sauce to coat for 1 minute to reheat and meld flavors. Stir in chopped cilantro if using. Remove from heat and sprinkle with reserved sliced green scallion tops and toasted sesame seeds.

Serve

Serve the pork immediately over steamed rice or in buns, spooning extra sauce over the top. Garnish with extra scallions and cilantro as desired.