Heat milk in saucepan, when hot, but not boiling, stir in flour. Stir until sauce begins to thicken, turn heat down. Add nutritional yeast and spinach. Stir constantly until thick and gooey, only about 3-4 minutes.
In separate pan, saute onion, garlic, until onion is tender. Stir in tomatoes, mushrooms, and spices. Saute until just done.
Preheat oven to 425 degrees .
Using a very sharp bread knife, split each pita bread so that you have two round flat pieces instead of one thick one. Place each piece of bread on a cookie sheet or pizza pan. Spread spinach sauce over each. Top with tomato mixture, being careful to avoid the juice.
Bake for 5 to 7 minutes. Watch closely so that the edges of the pita don't burn.