01 - In a large bowl, combine fish sauce, soy sauce, brown sugar, lemongrass, garlic, shallot, vegetable oil, black pepper, and chili (if using). Whisk until sugar dissolves completely.
02 - Add chicken thighs to the bowl and toss thoroughly to coat with marinade. Cover and refrigerate for at least 1 hour, or up to overnight for maximum flavor penetration.
03 - Preheat grill or grill pan over medium-high heat (approximately 400°F). Lightly oil the grates to prevent sticking.
04 - Take chicken out of marinade and allow excess liquid to drip off. Reserve marinade for basting if desired.
05 - Place chicken on hot grill and cook for 6-7 minutes per side, until internal temperature reaches 165°F and exterior develops nice char marks.
06 - Transfer grilled chicken to a clean plate and let rest for 5 minutes to redistribute juices. Slice if desired and serve with chopped cilantro, lime wedges, and sliced cucumber.