Bring water to boil, stir in rice, cover and simmer for 20 minutes or so, depending on the variety of rice selected. Add 2 cups hot milk, sugar and salt, and continue cooking, uncovered, over low heat until creamy. Stir in remaining milk as needed, add the butter and grated truffle, stir in the beaten egg, and serve steaming hot, topped with a truffle sliver and a slight grating of fresh nutmeg.