01 - Whisk together fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chopped chili in a large bowl until the sugar dissolves completely.
02 - Submerge chicken thighs in the marinade, ensuring each piece is thoroughly coated. Cover the bowl and refrigerate for at least 1 hour, preferably overnight for maximum flavor penetration.
03 - Heat grill or grill pan to medium-high temperature (approximately 375°F to 400°F). Lightly oil the grates to prevent sticking.
04 - Remove chicken from the marinade, allowing excess liquid to drip off. Discard used marinade.
05 - Place chicken on the preheated grill and cook for 6 to 7 minutes per side until internal temperature reaches 165°F and exterior shows attractive char marks.
06 - Transfer grilled chicken to a clean plate and let rest for 5 minutes to allow juices to redistribute. Slice as desired and serve with fresh cilantro, lime wedges, and cucumber slices.