Preheat oven to 400° F. Grease a 1 1/2-inch deep 9-inch tart pan with a removable bottom or a 9-inch round pie plate.

In a medium bowl, whisk together 1/2 cup flour and the flax seed. Stir in the quinoa and butter, mixing well. Spread into the tart pan or pie plate, pressing down on the bottom and along the sides. Bake for 20 minutes until the bottom and sides of the pie dries out a bit. Set aside.

Meanwhile, heat 2 tablespoons olive oil in a large (preferably 12-inch) heavy skillet over high heat. Add half the mushrooms and season with 1/4 teaspoon each salt and pepper. Cook until lightly browned and tender. Remove to a bowl and repeat with remaining mushrooms and 1/4 teaspoon each salt and pepper. Once tender, remove mushrooms to a bowl.

Add the leeks to the skillet and sauté until softened, about 3 minutes. Return the cooked mushrooms to the skillet along with the wine and cook until reduced by half, about 2 to 3 minutes. Stir in the flour and cook 2 minutes, stirring frequently. Add the broth, thyme and lemon zest and bring to a simmer. Simmer until slightly thickened, about 2 minutes. Taste for seasoning.

Place the sweet potatoes and root vegetables (if using) in a medium pot and add cold water to cover. Season with salt to taste and bring to a boil over high heat. Reduce the heat and simmer until a sharp knife slides easily into the vegetables. Drain and return vegetables to the pot. Stir in 2 tablespoons olive oil, cayenne, and 1/2 teaspoon each salt and pepper, mashing until fairly smooth. 

Transfer the mushroom mixture to the crust and spread the sweet potato mixture on top. Bake until pie is hot and top is lightly browned 30 to 35 minutes. Let cool 10 minutes. If using the tart pan, remove the sides and place on a platter, sprinkling with the parsley. If using a pie plate, just place on a platter, sprinkle with the parsley and serve.