01 - Whisk together fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chili (if using) in a large bowl until sugar dissolves completely.
02 - Add chicken thighs to the bowl, tossing thoroughly to coat evenly. Cover and refrigerate for at least 1 hour, preferably overnight for optimal flavor penetration.
03 - Heat grill or grill pan over medium-high heat (approximately 375°F). Lightly oil the grates to prevent sticking.
04 - Remove chicken from marinade, allowing excess liquid to drip off. Discard remaining marinade.
05 - Place chicken on preheated grill. Cook for 6 to 7 minutes per side until internal temperature reaches 165°F and exterior develops charred grill marks.
06 - Transfer grilled chicken to a clean plate. Let rest for 5 minutes to redistribute juices. Slice if desired and serve with cilantro, lime wedges, and cucumber.