If using butter lettuce: gently pull off the outer leaves and rinse them under cold water. Pat them dry with paper towels and arrange them on a large platter. Save the inner leaves for another use, or chop them and add them to the filling.
Heat the oil in a large skillet over medium heat. Add the onion and ginger and cook, stirring frequently, until soft, about 15 minutes. Add the garlic and cook for one more minute, stirring constantly.
Add the ground chicken and use a wooden spoon to break the meat apart. Cook, stirring occasionally, until no longer pink, about 10 minutes.
Add the soy sauce, fish sauce, brown sugar, lime zest, lime juice, diced lemongrass, and red pepper flakes, and bring the mixture to a boil. Reduce the heat and simmer, stirring occasionally, until the sauce is slightly thickened, about 15 minutes.
Off the heat, stir in the scallions, cilantro, mint, and nuts. Taste and adjust the seasoning if needed, adding more salt, sugar, lime juice, or red pepper flakes as desired.
Spoon the chicken mixture into the lettuce cups and serve with Sriracha or sambal, carrots, and more herbs if desired. Enjoy!
