01 - In a large mixing bowl, whisk together fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chopped chili if using.
02 - Add chicken thighs to the bowl and toss thoroughly to coat. Cover with plastic wrap and refrigerate for at least 1 hour, preferably overnight for maximum flavor penetration.
03 - Preheat grill or grill pan to medium-high heat. Lightly oil the grates to prevent sticking.
04 - Lift chicken from marinade and allow excess liquid to drip off. Reserve marinade for basting if desired.
05 - Place chicken on preheated grill. Cook for 6 to 7 minutes per side until internal temperature reaches 165°F and chicken develops a nice char.
06 - Transfer chicken to a serving plate and let rest for 5 minutes to redistribute juices. Slice if preferred and garnish with fresh cilantro, lime wedges, and sliced cucumber.