To prep the shiitake mushrooms, soak in 1 cup of warm water for 10 minutes, then drain, reserving the mushroom liquid. Chop the mushrooms and set aside.
Bake the frozen pie crust in the oven according to package instructions. While the crust bakes, heat a large skillet on medium heat on your stovetop. Add olive oil, then the onions and saute until onions are soft and translucent. Add in the button mushrooms, thyme and garlic granules; continue to cook until the mushrooms are soft, then add the spinach.
Once the spinach has wilted about 2 minutes, stir in the cream of mushroom soup, Dijon mustard, shiitake mushrooms and about 2 tbsp. of reserved mushroom liquid. Continue cooking until all excess liquid has reduced.
Season with garlic salt and pepper to taste. Then, spoon the mushroom-spinach mixture into the cooked pie crust, smooth the mashed potatoes over the top and sprinkle with the
Parmesan cheese. Return the pie to the oven (same temperature as stated on pie crust package) and bake for 10 minutes.
Remove from the oven and garnish with parsley and radish slices. Serve with a green or fresh fruit salad and your favorite white wine!