Finely chop the chicken thighs until they are a little coarser than ground beef. Heat the oil in a skillet or wok over medium-high heat until it shimmers. Add the garlic, ginger and white part of the scallions and stir-fry until fragrant, 20 seconds. Add the chicken and stir-fry until chicken is opaque and cooked through, about 8 minutes.
Reduce heat to medium; add green part of the scallions, soy sauce, rice vinegar, sweet chili sauce, water chestnuts and chili oil; stir-fry for 1 minute. Pour the chicken mixture into a serving bowl and fold in the cilantro.
Separate the leaves from the head of lettuce and wash thoroughly. Pat dry with paper towels. Tear the largest leaves in half lengthwise. Fill leaves with chicken mixture and drizzle with chili sauce, and eat them taco style.