01 - In a large bowl, combine fish sauce, soy sauce, brown sugar, lemongrass, garlic, shallot, vegetable oil, black pepper, and chili if using. Whisk until sugar dissolves completely.
02 - Add chicken thighs to the bowl and toss thoroughly to coat. Cover and refrigerate for at least 1 hour, or up to overnight for deeper flavor penetration.
03 - Preheat grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Remove chicken from marinade and allow excess liquid to drip off. Reserve any remaining marinade for basting if desired.
05 - Grill chicken for 6 to 7 minutes per side until cooked through with nice char marks. Internal temperature must reach 165°F for safe consumption.
06 - Transfer grilled chicken to a plate and let rest for 5 minutes to redistribute juices. Slice if desired and serve with chopped cilantro, lime wedges, and sliced cucumber.