Whisk together hoisin sauce, soy sauce, rice vinegar, and toasted sesame oil until smooth. Stir in cornstarch if a thicker sauce is desired. Set aside.

Heat vegetable oil in a large skillet over medium heat. Add ground chicken and cook for about 7-8 minutes until browned. Transfer chicken to a plate, leaving drippings in the skillet.

In the same skillet, add more vegetable oil and toss in finely chopped mushrooms and any optional vegetables. Sauté for about 4-5 minutes until tender.

Stir in water chestnuts, minced garlic, and grated ginger. Cook for 30 seconds until fragrant.

Return cooked chicken to the skillet with half of the sliced scallions. Pour in the sauce and stir until well combined and heated through.

Spoon the chicken filling into individual leaves of lettuce and garnish with remaining scallions. Serve with red pepper flakes or sriracha.