In a medium bowl, whisk together the low-sodium soy sauce, rice vinegar, sesame oil, grated ginger, minced garlic, and red pepper flakes (if using) to create the marinade. Add the sliced chicken breast to the marinade, ensuring all pieces are well coated. Stir in the cornflour until dissolved. Let the chicken marinate for at least 10 minutes, or up to 30 minutes for deeper flavor. Heat the vegetable oil in a large wok or a wide, non-stick frying pan over high heat until shimmering. Add the marinated chicken to the hot pan and stir-fry for 3-5 minutes, or until the chicken is browned and cooked through. Remove the cooked chicken from the pan and set it aside. Add the sliced onion and bell pepper to the same hot pan. Stir-fry for 3-4 minutes until they begin to soften but still retain some crunch. Add the sliced mushrooms and sugar snap peas to the pan. Continue to stir-fry for another 2-3 minutes until the vegetables are tender-crisp. Return the cooked chicken to the pan with the vegetables. Pour in the low-sodium chicken broth and the optional low-sodium oyster sauce. Bring the mixture to a simmer and cook for 1-2 minutes, allowing the sauce to thicken slightly and coat all the ingredients. Garnish generously with chopped spring onions. Serve your Healthy Asian Chicken Stir-Fry immediately with your choice of healthy accompaniment, such as brown rice or cauliflower rice.