If using iceberg lettuce: fill a large bowl with cold water. Place the head of lettuce on a cutting board; cut off the stem, then cut around the inner core of the lettuce and remove (do not cut all the way through the lettuce; just deep enough to remove the core). Discard the flimsy, ragged outer leaves and then place the head of lettuce in the bowl of water. Gently pull apart the leaves, then place them on a paper towel to dry. Cut any large leaves in half. Chill the lettuce cups in refrigerator until ready to serve. (If using butter lettuce, you won't have any issues pulling apart the leaves so this step is not necessary.)

Heat the oil in large sauté pan over medium heat. Add the onion and ginger and cook, stirring frequently, until soft, 4 to 5 minutes. Add the garlic and cook 1 minute more.

Add the ground chicken and turn the heat up to high. Cook, breaking up the meat with a wooden spoon, until partially cooked through, about 3 minutes. Add the soy sauce, fish sauce, brown sugar, lime zest, lime juice, and red pepper flakes and continue cooking, stirring frequently to break up the meat, until the chicken is cooked through, 5 to 6 minutes more. Off the heat, stir in the scallions, cilantro, mint, and nuts. Taste and adjust seasoning if necessary. Spoon into lettuce cups and serve with optional garnishes.