Gnocchi with Mushroom Sauce

Name: Gnocchi with Mushroom Sauce
Ingredients: 1 1/2 pounds russet potatoes (about 2 medium); 1 large egg yolk; 1/2 teaspoon salt; 1/4 teaspoon ground black pepper; Pinch of grated nutmeg; 7 tablespoons (about) all purpose flour; 2 tablespoons (1/4 stick) butter; 2 tablespoons olive oil; 12 ounces fresh shiitake mushrooms, stemmed, sliced; 1/2 cup sliced shallots; 1 3/4 cups chicken stock or canned low-salt chicken broth; 1 tablespoon chopped fresh sage; 3 cups coarsely chopped arugula; 1/2 teaspoon (about) white truffle oil (optional)*; 1/3 cup grated Parmesan cheese; *Available at Italian markets, specialty foods stores and some supermarkets.