Preheat the oven to 350 F. To make the filling; quarter the pumpkin, removing the strings and seeds and place, flesh side down, on an oiled baking sheet. Bake for 35 minutes, or until tender. Peel and puree flesh. Add egg yolk, cheese, sage, nutmeg and salt and pepper to taste. Arrange one wanton skin on work surface. Add 1 teaspoon filling, mounding it in center of square and brush border with egg wash. Top with another wanton skin, pressing dough around filling to remove any air bubbles and transfer to a lightly floured baking sheet. Cover with dampened towels while preparing remaining raviolis. In a saucepan of boiling salted water cook the ravioli until they are just tender. In a saucepan set over moderate heat melt the butter, add the sage and cook, stirring, 1 minute. Add the cream and simmer until lightly thickened. Drain the pasta and transfer to a serving bowl. Add sauce and toss to combine. Garnish with sage leaves.