How to Make It
Preheat the oven to 400°. Position a rack in the top third of the oven. In a food processor, pulse the chicken until coarsely ground; transfer to a bowl. Add the fish sauce, shallots, garlic, lemongrass, chopped cilantro and mint, cornstarch, salt and pepper and mix with your hands.
Line a large, rimmed baking sheet with parchment paper. Spread the sugar on a plate. Using slightly moistened hands, roll the chicken mixture into 1 1/2-inch balls. Roll the meatballs in the sugar until they are evenly coated. Transfer the meatballs to the prepared baking sheet and bake them for 15 minutes, until they are lightly browned and cooked through.
Meanwhile, arrange the lettuce, cilantro and mint leaves, cucumber and onion on a platter. Transfer the meatballs to the platter and serve with chili sauce.