Finely dice the cremini mushrooms and water chestnuts to a similar consistency as the ground chicken.
Mince the garlic and ginger, and thinly slice the green onions, keeping the white and green parts separate.
Heat the toasted sesame oil in a large skillet over medium-high heat.
Add the ground chicken to the skillet, seasoning with sea salt and black pepper, and cook until browned and crumbled.
Stir in the diced mushrooms, water chestnuts, minced garlic, ginger, and the white parts of the green onions.
Sauté for 4-5 minutes until the mushrooms have softened and released their moisture.
Whisk together the hoisin sauce and tamari in a small bowl, then pour over the chicken mixture.
Toss everything together for 1-2 minutes until the sauce is thickened and coats the ingredients evenly.
Remove from heat and stir in the green parts of the onions.
Spoon the warm chicken and mushroom filling into the butter lettuce leaves and serve immediately.