01 - In a large bowl, whisk together fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chili if using. Ensure sugar is fully dissolved.
02 - Add chicken thighs to the bowl and turn to coat completely. Cover and refrigerate for at least 1 hour, preferably up to overnight for maximum flavor penetration.
03 - Prepare grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Take chicken from marinade and let excess liquid drip off. Reserve marinade for basting if desired.
05 - Place chicken on hot grill and cook for 6 to 7 minutes per side. Flip once and continue cooking until internal temperature reaches 165°F and exterior shows nice char marks.
06 - Transfer chicken to serving plate and let rest for 5 minutes to redistribute juices. Slice if desired. Garnish with fresh cilantro, lime wedges, and cucumber slices.