Preheat the oven to 400 F.
Make the topping by placing all the Topping Ingredients into a small bowl, whisk well, set aside.
Cut and place the potatoes into a medium-sized pot, sprinkle with some sea salt. Boil until tender, approximately 15 to 20 minutes. Go to Step 4.
In the meantime, add the cremini mushrooms to a nonstick skillet with the tamari, sauté for several minutes, then add all the remaining Mushroom Ingredients, sauté for 5 to 7 minutes to allow the mushrooms to release their moisture and reduce in size, then take off the burner and set aside. Note: If there is extra mushroom liquid left over after the mushrooms sauté and sit, discard – or feel free to save for another dish. We had less than a teaspoon of liquid that did not sauté off.
After the potatoes are boiled until tender, drain off all the water, mash with a potato masher, then add all the remained Mashed Potato Ingredients into the pot with the mashed potatoes and mash/beat with an electric beater until smooth and creamy.
Taste test and adjust seasonings to fit your personal preference. Note: keep in mind the seasonings will intensify as the casserole bakes.
Fold in the sautéed mushrooms and (optional) warmed frozen vegetable mix to the mashed potatoes.
Scoop out the mashed potato mixture into a small casserole pan, smooth out evenly, then sprinkle the Topping Mix over top.
Bake in a 400 F oven for 20 minutes, then switch the oven to high broil and broil for 3 to 5 minutes until the top is golden brown.
Remove from the oven, allow to set for 5 minutes, then sprinkle with freshly chopped parsley. Serve.