Pumpkin Gorgonzola Pasta


1 pound penne pasta
2 tablespoons olive oil
12 ounces mushrooms, sliced
2 sweet onions, halved and then sliced vertically
4 cloves garlic, minced
1 1/2 teaspoons chopped sage
1 12-ounce can light evaporated milk
1 1/2 tablespoons cornstarch or arrowroot
3 ounces crumbled Gorgonzola cheese
1/2 cup canned pure pumpkin
1/4 teaspoon ground nutmeg
salt and pepper to taste