Heat 2 tablespoons of the olive oil in a medium-size sauté pan over medium heat. Add the shallots and cook, stirring, for 30 seconds. Add the mushrooms and cook, stirring, for 2 minutes, then season with salt and pepper. Add the garlic and prosciutto, and cook, stirring, for 1 minute. Remove from the heat and pour into a medium-size mixing bowl. Stir in three-fourths cup of the cheese and let cool completely.
Place the water in a large saucepan, salt it and add the remaining 1 tablespoon olive oil. Bring to a boil. With a sharp knife, cut the pasta sheet into sixteen 3-inch squares.
Using your fingers, lightly wed the edges of each square.
Spoon 1 tablespoon of the mushroom mixture into the center of 8 of the squares. Place the remaining pasta squares over the fillings and press the edges with the tines of a fork to seal.
Drop the ravioli into the boiling water and cook until they rise to the surface, 3 to 4 minutes, then cook for another 2 minutes. With a slotted spoon, remove the ravioli and drain on paper towels.
Toss the ravioli with the truffle oil in a large mixing bowl. Season with salt and pepper.
Heat the butter in a large sauté pan over medium heat. Add the sage leaves and cook for 30 seconds. Add the ravioli to the pan and cook, stirring, for 1 minute. Serve in a shallow bowl, garnished with the remaining one-fourth cup cheese and the chives.
Makes 4 servings