01 - In a large mixing 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. Cover with plastic wrap and refrigerate for at least 1 hour, preferably up to overnight for optimal flavor penetration.
03 - Prepare a grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
04 - Remove chicken from marinade, allowing excess to drip off. Place on hot grill and cook for 6 to 7 minutes per side until cooked through with nice char marks. Internal temperature should reach 165°F.
05 - Transfer grilled chicken to a serving plate and let rest for 5 minutes to redistribute juices. Slice if desired and serve with fresh cilantro, lime wedges, and sliced cucumber.