01 - Combine fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chili (if using) in a large mixing bowl. Whisk until the sugar has completely dissolved.
02 - Add chicken thighs to the bowl and toss thoroughly to ensure even coating. Cover and refrigerate for at least 1 hour, or up to overnight for optimal flavor penetration.
03 - Prepare a grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Take chicken out of the marinade and allow excess liquid to drip off. Reserve the remaining marinade for basting if desired.
05 - Place chicken on the preheated grill and cook for 6 to 7 minutes per side. Cook until the meat reaches an internal temperature of 165°F and develops a nice char.
06 - Transfer the grilled chicken to a serving plate and let rest for 5 minutes to allow juices to redistribute. Slice if desired, then serve topped with fresh cilantro, lime wedges, and sliced cucumber.