Pat the sliced beef belly dry with paper towels to remove excess moisture. If slices are thick, partially freeze the slab for 20–30 minutes and then slice across the grain to 2–3 mm thickness.

Make marinade

In a large bowl combine soy sauce, mirin, brown sugar, sesame oil, grated ginger, grated garlic, gochujang (if using), and black pepper. Stir until sugar dissolves.

Marinate

Add the sliced beef belly to the marinade and toss to coat evenly. Cover and refrigerate for 30 minutes to 2 hours. For best flavor without over-tenderizing, 1 hour is recommended.

For Cooking & Garnish

Preheat pan

Remove beef from refrigerator about 10 minutes before cooking to take the chill off. Heat a large heavy skillet or cast-iron pan over medium-high heat until very hot. Add 2 tsp neutral oil and swirl to coat.

Cook beef in batches

Working in a single layer and not crowding the pan, add portions of the marinated beef belly in a quick, even layer. Cook 30–45 seconds without moving to develop a sear, then flip or stir and cook another 30–45 seconds just until cooked through and edges caramelize. Transfer cooked portions to a plate and repeat with remaining beef. Total cook time for each batch is about 1–1.5 minutes.

Finish

If there is excess marinade left in the bowl, pour it into the hot pan and simmer 1–2 minutes to reduce and thicken slightly (optional). Return the cooked beef to the pan and toss quickly to glaze, heating through but not overcooking.

Garnish and serve

Transfer the sliced beef belly to a serving platter. Sprinkle with thinly sliced scallions and toasted sesame seeds. Serve immediately with lime or lemon wedges to squeeze over before eating.