Add the red pepper flakes (if using), white and light green parts of the sliced green onion (reserve dark green parts for garnish), garlic, ginger, and water chestnuts. Stir together and sauté for 2 minutes. Add hoisin ad soy sauce, rice wine vinegar, and chicken broth. Stir to combine. Secure pressure cooker lid and cook on high pressure for 3 minutes. Release pressure as soon as the cooking cycle is complete.