In a mixing bowl, combine the finely chopped lemongrass, minced garlic, and red chilies. Add fish sauce, soy sauce, sugar, vegetable oil, and black pepper. Stir well until all ingredients are thoroughly blended.

Place the boneless chicken thighs or breasts into the bowl of marinade, ensuring each piece is generously coated. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes.

While the chicken marinates, preheat your grill or a non-stick skillet over medium-high heat.

Remove the chicken from the marinade and let any excess drip off. Place it onto the hot grill or skillet. Cook for 6-7 minutes on each side.

Transfer the cooked chicken to a cutting board and let it rest for about 5 minutes.

Slice the chicken into strips for easier serving and arrange the pieces on a platter.

Garnish with freshly chopped cilantro and serve alongside lime wedges.