Heat oven to 375 degrees F. Coat 10-inch springform pan with nonstick vegetable cooking spray.
Heat oil in medium-size nonstick skillet over medium heat. Add onion and garlic; sauté for 5 minutes. Add sausage slices; sauté until cooked through, about 10 minutes. Remove skillet from the heat.
Combine ricotta, eggs, provolone, prosciutto, Parmesan, salt and pepper in large bowl. Stir in the sausage mixture.
Roll out both piecrusts on lightly floured surface with lightly floured rolling pin to two 14-inch rounds. Place one round in bottom of prepared pan. With fingers, lightly press crust into bottom and up sides of pan, without stretching dough.
Spoon cheese mixture into pastry-lined pan. Place second crust on top of cheese; trim edges of pastry even with rim of pan. Brush pastry with beaten egg yolk.
Bake on the bottom oven rack in 375 degrees F oven for 45 minutes. (If the top begins to brown too quickly, cover with sheet of aluminum foil.) Turn oven off; let the pie continue to cook in the oven for another 15 minutes or until a knife inserted in the center comes out clean. Makes 12 servings.