Cut the chicken thighs in half and flatten slightly.
Remove the outer leaves of the lemongrass. Keep only the white part and chop finely. With a mortar or small food processor, crush the lemongrass.
In a bowl, combine the lemongrass, half the nuoc cham sauce and the chicken. Cover and let marinate in the refrigerator for 2 hours, ideally. When short on time, marinate for 30 minutes on the counter. If you plan to grill the chicken, add the oil in the marinade.
Thread one piece of chicken per skewer. Brown the chicken on the grill or a ridged skillet in the peanut and sesame oil, 3 to 4 minutes on each side.
When serving, place the skewers on the lettuce leaves and sprinkle with the peanuts. Serve with the remaining sauce.