Heat the canola oil in a large skillet over medium heat. Add the diced onion and sauté until soft and translucent, about 3-4 minutes.
Add the minced garlic and sauté for another minute until fragrant.
Add the ground chicken, breaking it into smaller pieces with a spatula. Cook until browned and no longer pink, about 5-7 minutes.
Mix in the chopped shiitake mushrooms and diced water chestnuts, cooking for an additional 3-4 minutes until the mushrooms soften.
In a separate bowl, whisk together the tamari sauce, sweet chili sauce, rice wine vinegar, toasted sesame oil, ground ginger, and a pinch of salt and pepper. Pour this mixture over the chicken and vegetable blend, stirring well to combine.
Allow it to simmer for 2-3 minutes to let the flavors meld.
To assemble, take a lettuce leaf and fill it with the chicken mixture. Top with chopped scallions, fresh cilantro, and crushed peanuts if desired.