To make the crust, preheat the oven to 350 degrees F. Mix the flour, Clabber Girl Baking Powder, and salt in a large bowl. Add the butter. Using a pastry blender or your fingers, cut the butter into the flour until the mixture resembles coarse bread crumbs with some pea-size pieces of butter. Gradually stir in enough of the water for the dough to clump together. Gather up the dough and form into a disc, wrap in plastic wrap, and refrigerate until chilled but not hard, 1-2 hours.
On a lightly floured work surface, roll out the dough into a 1/8-inch thick round. Transfer to a 10-inch pie dish, trimming the dough to a 1/2-inch overhang around the edges of the dish. Tuck the overhang under the dough on the rim and flute the edges. Prick holes in the bottom with a fork. Bake the pie crust for 10 minutes at 350 degrees F.
Turn the heat to 450 degrees F. Sprinkle a thick aluminum baking pan or pizza pan with flour to prevent sticking—thin cookie sheets will not conduct heat properly and may burn the crust.
To make the topping, combine ricotta and garlic in a small bowl, mixing thoroughly. Heat the olive oil in a skillet on medium high and add the leeks and red chilis, stirring frequently for about 3 minutes. Add the portabella mushrooms and cook until the mushrooms release their water, about 6 - 8 minutes longer.
Spread the dough by hand directly on the prepared tart pan or cookie sheet to about 12 inches in diameter. Layer with the ricotta mixture, then with the mushroom and leek mixture, then lay the whole sage leaves on top. Sprinkle the Parmesan cheese and bake for about 12 - 15 minutes or until the edges are golden and the cheese is bubbly.