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.
02 - Add chicken thighs to the bowl and toss to coat thoroughly. Cover and refrigerate for at least 1 hour, or overnight for deeper flavor penetration.
03 - Prepare a grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Remove chicken from the marinade, allowing excess liquid to drip off. Reserve the marinade for basting if desired.
05 - Place chicken on the grill and cook for 6 to 7 minutes per side, until internal temperature reaches 165°F and exterior develops nice char marks.
06 - Transfer grilled chicken to a plate and let rest for 5 minutes before slicing. Serve garnished with fresh cilantro, lime wedges, and cucumber slices.