Boil the shells in large pot of salted boiling water until firm. They will have to be baked later on.
Brown the diced onion in the butter, add the Prosciutto di San Daniele thinly sliced, cook for several minutes. Place the Montasio cheese in a bowl, add the Prosciutto di San Daniele and onion mixture. Salt and pepper to taste. Mix in the beaten eggs until you get a nice smooth mixture.
Fill the pasta shells with the mixture and place them in a buttered baking dish. Dot with more butter and dust the Parmigiano Reggiano cheese over the tops of the shells.
Bake in a preheated oven at 350°F for 15 minutes. Serve immediately.
Serves 4
