In a medium-sized bowl, combine the finely chopped lemongrass, minced garlic, and chopped red chilies with the fish sauce, soy sauce, sugar, vegetable oil, and black pepper. Stir well for about 2 minutes until the sugar dissolves and all the ingredients are mixed into a fragrant marinade.

Add the boneless and skinless chicken thighs or breasts to the bowl with the marinade. Use your hands to thoroughly coat each piece, ensuring every surface is drenched with the aromatic mixture. Cover the bowl with plastic wrap and refrigerate for at least 30 minutes; for best results, marinate for up to 2 hours.

While the chicken marinates, preheat your grill or non-stick skillet over medium-high heat for about 5 minutes.

Once hot, remove the chicken from the marinade, letting any excess drip off. Place the chicken in the grill or skillet, cooking for about 6-7 minutes on one side, until it turns a beautiful golden brown. Flip the chicken and repeat on the other side, cooking until the internal temperature reaches 165°F (75°C).

After cooking, remove the chicken from the heat and transfer it to a plate. Allow the chicken to rest for about 5 minutes.

Slice the rested chicken into strips, showcasing the juicy, tender meat. Arrange the slices on a serving platter.

To complete the dish, sprinkle freshly chopped cilantro over the sliced chicken and serve with lime wedges on the side.