Peel and quarter potatoes. Place in a medium or large size pot with water to cover. Add garlic, celery, parsley, peppercorns, onion, and bay leaf. Bring to a boil, cover, and simmer over medium-low heat for 15 to 20 minutes or until potatoes are very tender.
To Make Filling: While potatoes are cooking, in a large skillet heat 1 tablespoon oil and saute onion and garlic. Saute for 1 minute over medium heat, then add mushrooms and saute for 2 minutes. Crumble tofu in chunks into the skillet and saute briefly, mixing well. Stir in barbecue sauce, yeast, gravy mix, thyme, paprika, and tamari. Mix well and saute, stirring frequently, for 20 minutes over medium heat.
Transfer potatoes from water to a large bowl, reserving 3 1/2 cups of the remaining stock. Add miso, oil, and 3/4 to 1 cup of the potato stock to the potatoes a little at a time, mashing potatoes as you add the stock. Add only enough water to moisten potatoes adequately. Do not over moisten, this potato mixture will be the crust covering of the casserole.
Add corn and spinach to filling mixture and mix well. Spoon filling into an oiled, shallow ovenproof casserole dish. Pat down with back of a large spoon. Spread potato crust evenly over filling, smoothing top with a spoon or spatula. Dust evenly with paprika. Bake for 30 to 40 minutes, or until crust is golden.
While casserole bakes, prepare gravy. Heat oil in a large frying pan. Add flour and yeast, stir with a whisk over medium heat to form a paste. Slowly stir in 2 1/2 cups of reserved potato water, whisking as you stir to allow gravy to thicken. Stir in instant gravy mix and continue whisking until gravy is thick and smooth; add additional potato water, if necessary. Serve casserole with crust on the bottom and filling on top. Spoon gravy