Heat peanut oil in a large skillet over medium heat. Add diced onion and cook 3-4 minutes.
Add ginger and garlic; cook 30 seconds.
Add ground chicken. Cook 5-6 minutes, breaking it up until fully cooked.
Stir in mushrooms and water chestnuts.
In a small bowl, mix soy sauce, vinegar, brown sugar, and hoisin/Sriracha if using. Pour over chicken and stir well.
Simmer 3-5 minutes until thickened.
Top with diced green onions.
Serve in lettuce cups and enjoy!