Step 1

Purée hoisin, gochujang, garlic, ½ tsp. pepper, ¼ tsp. vinegar, and ⅔ cup water in a blender until smooth. Place lamb chops in a baking dish and pour marinade over; turn to coat. Let sit at room temperature at least 15 minutes.

Step 2

Meanwhile, arrange scallions on a rimmed baking sheet and drizzle with grapeseed oil; turn to coat. Season with salt and pepper; set aside.

Step 3

Heat sesame oil in a large high-sided skillet over medium-high; add 2 Tbsp. butter. Working in 2 batches, remove lamb chops from marinade, letting excess drain back into dish, and cook in a single layer until golden brown, about 3 minutes per side for medium-rare. Transfer to a platter; set marinade aside.

Step 4

Add onion to drippings in skillet and cook (still over medium-high), stirring occasionally, until golden brown, about 10 minutes. Add reserved marinade and bring to a simmer. Reduce heat to medium and cook, stirring occasionally, until thickened, about 2 minutes. Stir remaining ⅛ tsp. vinegar into sauce and remove from heat.

Step 5

Drizzle ½ cup sauce over lamb chops. Let remaining sauce in skillet cool 1 minute, then transfer to a blender. Add remaining 1 Tbsp. butter and 2 Tbsp. water and purée until smooth. Transfer to a small bowl and set dipping sauce aside.

Step 6

Wipe out skillet (or use a medium skillet if you prefer) and heat over medium-high. Cook reserved scallions, turning often, until charred in spots, about 3 minutes.

Step 7

Arrange scallions on platter next to lamb chops and sprinkle sesame seeds over lamb. Serve with reserved dipping sauce alongside.