On a lightly floured surface, roll 1 ball of dough into an 8-inch round. Dust a pizza peel with semolina and set the dough round on it. Lightly brush the dough with olive oil. Scatter 1/2 cup of the Fontina cheese and 1/4 cup of the mozzarella on the dough, then top with one-fourth of the asparagus and 1/4 cup of the prosciutto. Slide the pizzetta onto the hot stone. Bake for about 6 minutes, or until it is crisp and browned on the bottom and bubbling on top. Using a large spatula, transfer the pizzetta to a work surface and cut it into quarters. Sprinkle the pizzetta with the sorrel and lemon juice and serve. As soon as 1 pizzetta is in the oven, prepare the next. Slide it into the oven when the first comes out.