Pat the chicken thighs dry and season with salt and pepper. Heat a 4½ - quart Dutch oven over medium heat, add the olive oil and heat through. Add the sugar to the oil, stirring constantly until the sugar has dissolved.
Cut the lemongrass stalks in half lengthwise and then smash them using the flat side of a chef knife.
Add the chicken skin side down in a single layer and sauté until browned on both sides, about 2 minutes each side. The chicken skin should be slightly caramelized. Remove from the pot and set aside.
Add the onion and sauté until soft and translucent, stirring occasionally, about 6 minutes. Add the garlic, ginger and lemongrass and sauté for an additional 2 minutes. Then add the chicken stock and bring to a simmer.
Return the chicken to the pan, skin side up, and bring to a boil. Cover tightly and bake for 1 hour, until meat is fork tender. Remove the pot from the oven.
Discard the lemongrass. Stir in the fish sauce and lemon juice then season the sauce with salt and pepper, to taste.
Serve the chicken with a side of rice and topped with the remaining braising liquid.