01 - In a large bowl, whisk together fish sauce, soy sauce, brown sugar, minced lemongrass, garlic, shallot, vegetable oil, black pepper, and chopped chili if using. Stir until sugar dissolves completely.
02 - Add chicken thighs to the bowl and turn to coat thoroughly with marinade. Cover container and refrigerate for at least 1 hour, preferably overnight for maximum flavor penetration.
03 - Heat grill or grill pan over medium-high heat. Lightly oil grates to prevent sticking. Ensure temperature reaches approximately 400°F.
04 - Remove chicken from marinade, allowing excess liquid to drip off. Reserve marinade for basting if desired.
05 - Place chicken on preheated grill. Cook 6 to 7 minutes per side until internal temperature reaches 165°F and exterior develops nice char marks. Total cooking time approximately 12 to 14 minutes.
06 - Transfer grilled chicken to serving plate. Let rest 5 minutes to allow juices to redistribute. Slice if desired and garnish with fresh cilantro, lime wedges, and sliced cucumber.