Position the rack on the bottom of the oven, and preheat the oven to 375 degrees F.
Heat 1 tablespoon of oil in a heavy large frying pan over medium heat. Add the sausages and saute until golden brown, breaking the sausage into pieces, about 5 minutes. Add the garlic and saute until fragrant, about 1 minute. Transfer the mixture to a small bowl and set aside to cool.
Meanwhile, heat 1 tablespoon of oil in the same frying pan over medium heat. Add the spinach and cook until the spinach wilts and the juices evaporate, stirring often, about 10 minutes. Cool to room temperature. Squeeze the spinach to drain as much liquid as possible.
Into a large bowl, add egg yolks and beat lightly. Stir in the ricotta, mozzarella, and 1/3 cup of Parmesan cheese. Add the sausage, the spinach and prosciutto to the mixture and stir to combine.
Roll out larger piece of dough on a lightly floured work surface to a 17-inch round. Transfer the dough to a 9-inch springform pan. Trim the dough overhang to 1 inch. Spoon the ricotta mixture into the dough-lined pan. Roll out the remaining piece of dough into a 12-inch round. Place the dough over the filling. Pinch the edges of the doughs together to seal, then crimp the dough edges decoratively. Brush the beaten 1 large egg over the entire pastry top. Sprinkle the remaining 2 tablespoons Parmesan over the top. Bake on the bottom shelf until the crust is golden brown, about 1 hour.
Let stand 15 minutes. Release the pan sides and transfer the pizza to a platter. Cut into wedges and serve.
Yield:2 dough pieces (enough for 1 Pizza Rustica)
Blend the flour, the butter, the shortening and salt in a food processor until the mixture resembles coarse meal. Blend in the eggs. With the machine running, add the water 1 tablespoon at a time until the dough forms. Gather the dough into a ball. Divide the dough into 2 pieces, with 1 piece twice as large as the second piece. Flatten the dough pieces into disks. Wrap in plastic wrap and refrigerate until the dough is firm enough to roll out, about 30 minutes.