Season chicken pieces with ½ tsp salt.
In a small bowl stir together the sauce ingredients: hoisin sauce, soy sauce, sesame oil, garlic, ginger and rice vinegar. Set aside.
Heat avocado oil in a large skillet over medium heat. Add chicken to hot skillet. Cook, flipping once, until cooked through, about 3-4 minutes.
Add the sauce ingredients and simmer for 2 minutes. In the meantime whisk together the cornstarch with the cold water, being sure there are no lumps. Add the cornstarch mixture to the sauce, reduce to a simmer and continue to cook, stirring, until thickened.
Stir in cashews and most of the green onion, reserving a big handful for garnish at the end.
Serve in cold lettuce cups. Top with remaining chopped green onions if desired.