In a mixing bowl, combine finely minced lemongrass, crushed garlic, diced shallots, fish sauce, soy sauce, and sugar to create a fragrant marinade. Add the chicken thighs, ensuring they are fully coated in the mixture. Cover and let the chicken marinate for at least 15 minutes at room temperature, or refrigerate for up to overnight.

While the chicken marinates, preheat your grill to medium heat, around 350°F (175°C). Ensure the grill grates are clean and oiled to prevent sticking.

Once preheated, place the marinated chicken thighs skin-side down on the grill. Sear them for about 3-5 minutes, leaving them undisturbed.

After achieving a nice sear, flip the chicken thighs carefully and continue grilling for an additional 8-10 minutes. Use a meat thermometer to check that the internal temperature reaches 165°F (74°C).

Once the chicken is cooked through, transfer it to a platter and allow it to rest for a few minutes. Slice and serve with steamed jasmine rice or vermicelli noodles, and a side of nuoc cham.