Trim and discard the tough outer layers of the lemongrass; use only the pale lower 3–4 inches. Finely chop the white parts. Roughly chop the red chilies (remove seeds for less heat). Peel garlic and chop along with the ginger.

Make marinade

In a blender or food processor, combine chopped lemongrass, chilies, garlic, ginger, light soy sauce, fish sauce, brown sugar, lime juice, vegetable oil, black pepper and kosher salt. Pulse until you have a coarse paste — smoothness is not necessary; you want some texture. Scrape down sides as needed.

Marinate chicken

Pat chicken thighs dry with paper towels. Place thighs in a large bowl or resealable bag and pour the marinade over them, massaging to coat thoroughly so the marinade gets under any skin pockets. Refrigerate for at least 2 hours, ideally overnight (8–12 hours) for maximum flavor. If short on time, allow at least 30 minutes at room temperature before grilling.

Grilling & Finish

Preheat and oil grill

Preheat a grill or grill pan to medium-high heat (about 200–230°C / 400–450°F). Clean grates well. Lightly brush the grill grates with neutral oil or brush oil on the chicken skin to prevent sticking.

Grill the chicken

Place thighs skin-side down on the hot grill. Cook 5–7 minutes without moving to get good char and to render fat. Flip and cook an additional 5–7 minutes, turning once or twice, until an instant-read thermometer inserted into the thickest part reads 74°C (165°F) and juices run clear. Total grilling time will be about 10–15 minutes depending on thickness and grill temperature. If pieces brown too quickly, move to a cooler part of the grill and cook until done.

Rest

Transfer grilled thighs to a cutting board, tent loosely with foil and rest for 5 minutes to allow juices to redistribute.

Slice and garnish

Slice thighs across the grain. Arrange on a platter, sprinkle with chopped cilantro and extra chili slices if desired. Serve with lime wedges for squeezing over the chicken.