In a large bowl, combine the minced lemongrass, garlic, fish sauce, soy sauce, brown sugar, black pepper, and chili flakes (if using). Mix well to create a marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Heat the vegetable oil in a large skillet or grill pan over medium-high heat.
Remove the chicken from the marinade, allowing excess marinade to drip off. Place the chicken in the hot skillet.
Cook the chicken for about 6-7 minutes on each side, or until fully cooked and golden brown. The internal temperature should reach 165°F.
Once cooked, remove the chicken from the skillet and let it rest for a few minutes before slicing.
Serve the chicken garnished with chopped cilantro and lime wedges on the side.