Whisk together cornstarch and water. Slowly add to the sauce, whisking all the while. Once it comes to a boil turn off the heat.
Make the Stir Fry
Get your wok or pan smoking hot. Add peanut oil or coconut oil. Stirfry shiitake mushrooms for 2 minutes. Push to the side of the wok.
Add bell pepper, half of the ginger and half of the garlic stir-fry 1 minute. Push to the side.
Add scallions stir-fry a minute add the remaining ginger and garlic and Bok Choy. Stir another minute until Bok Choy is just beginning to wilt. Add soy sauce, rice vinegar, and sesame oil. Mix in the crispy tofu.