Saute onions, mushrooms and tempeh in a large pan until onions are soft.
Add peas and carrots.
In a bowl, whisk together remaining ingredients, except mashed potatoes.
Pour mixture into the vegetable mixture, stirring constantly.
Bring to a boil and let simmer until thickened. Remove from heat.
Preheat oven to 350 degrees F.
Pour vegetable mixture into a baking dish and spread the mashed potatoes on top.
Bake for 30 minutes. Let sit for 10-15 minutes before serving.