Marinating the tempeh (you can do this first before you prep everything else or the day before for maximum absorption and flavor):

Cut the block of tempeh in half width wise so it makes two shorter blocks.

Boil or steam in a pot of water for about 20 minutes. This cuts the bitterness from the tempeh.

Meanwhile, make marinade by combining everything in a large container or large ziplock bag.

Once the tempeh is done, allow to cool. Then cut into thin slices, about 1/8 inch thick. Using a very sharp knife will make this easier.

Add thinly sliced tempeh into the marinade and allow it to soak for at least 4-6 hours, overnight is best.

If using mushrooms then put thin slices right into marinade.

Once tempeh (or mushrooms) is ready:

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Preheat oven to 350 F (175 C). 
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Take tempeh slices out of the marinade (save it!) and spread them out in a thin layer in a 9 x 13 baking dish. Do this with mushroom slices too but no need to save the marinade.

Gently pour about 1/4 cup of the marinade over the tempeh slices.

Bake for 30-35 minutes or until the edges are brown.

Meanwhile you can prep the tomatoes, red onion, lettuce and make tzatziki if you haven't yet. If you use potatoes then make sure to make them as well, they take a while to bake so you may want to do this first.

Once tempeh (or mushrooms) is done, remove from oven and cover with foil to keep warm. Set aside.

Warm the pita bread in a damp towel for 2-3 minutes in the microwave to soften.

Build your gyros with the tempeh gyro slices, potatoes (if using), tomatoes, red onion and lettuce if preferred.

Drizzle tzatziki over the top and devour!