Put all the ingredients for the sauce into a tupperware container and seal. Shake well until cornstarch has dissolved and set aside. You might need to break up the lumps by squashing them with a spoon.
In a large pan over medium high heat, add canola oil. When the oil is hot, add garlic and seitan and cook for 4-5 minutes, until seitan is golden brown.
Add onion, red bell pepper and shiitake mushrooms and cook for 4-5 minutes, until vegetables are tender but still yielding a crunch.
Add spinach and cook for 1 minute.
Shake the tupperware container with the sauce and pour into the pan. Stir well and cook for 1 minute.
Turn the heat off and serve with rice.
