Into a food processor, add cilantro, garlic, brown sugar, fish sauce, soy sauce, lemongrass. Puree until chunky. Rub chicken thighs with mixture. Place in a zip-top bag and marinate for at least 2 hours to overnight.
Heat grill to medium-high heat.
Remove chicken from marinade and grill for 5 to 6 minutes per side, until cooked through and juices run clear.
Serve with cilantro, red chili slices and lime wedges.