Prepare the vegetables by chopping them into bite-sized chunks. Mince the garlic and ginger. Gather the remaining ingredients because this dish cooks fast.
Make the sauce by whisking together the cornstarch and 2 tbsp of water until it's lump-free. Add the remaining water, soy sauce, brown sugar, rice vinegar, hoisin, sesame oil, and red pepper flakes. Set it aside for now.
Heat a large pan to medium heat. Add the cooking oil. Once hot, add the white onions, peppers, broccoli, and mushrooms. Stir regularly as they cook until they are soft (5 minutes).
Add the garlic, ginger, and whole dried chilies. Stir occasionally as they cook for 2 minutes.
Give the sauce another stir and then pour it into the pan. Lower the heat to medium-low and let the veggies cook for 5 more minutes.
Take the pan off of the heat and stir in the peanuts and green onions. Enjoy over rice or as-is!