Put half of the lemongrass into a bowl with the fish sauce, sugar, half of the jalapenos, garlic, and chicken. Toss to combine and let marinate in the refrigerator for at least 4 hours (preferably overnight).
Heat the oil in a large skillet or a wok over medium-high heat. Add the chicken and cook, until browned. Add the reserved jalapenos, lemongrass and onion. Cook until the onion is softened. Pour in the stock, reduce the heat to low, cover and cook, about 5 minutes, until the chicken is cooked through. Add lime juice.
Meanwhile, toast the rice in a small skillet over medium-high heat, until golden brown. Remove from heat. Using a mortar & pestle grind the rice into a coarse powder. Alternatively, you can use a clean coffee grinder for this. Set aside.
Remove the lid, increase the heat, and continue to cook until the liquid is thickened. Sprinkle with toasted rice powder and cilantro and serve with rice and lime wedges on the side.