In a 6 quart kettle bring 5 quarts salted water to a boil for pasta.
In a 10 to 12inch nonstick skillet heat 1 tablespoon oil over moderately high heat until hot but not smoking and saute mushrooms with Worcestershire sauce, Sherry, and salt and pepper to taste, stirring, until liquid mushrooms give off is evaporated and mushrooms begin to brown, about 10 minutes.
In a food processor puree mushroom mixture with garlic, pine nuts, Parmesan, and remaining 3 tablespoons oil. Add parsley and blend until parsley is chopped fine. Pesto keeps, surface covered with plastic wrap, chilled, 1 week.
Cook pasta in boiling water until al dente. Reserve 1 cup cooking water and drain pasta.
In a large bowl whisk together pesto and 2/3 cup reserved hot cooking water. Add pasta and toss well, thinning pesto, if necessary, with remaining reserved cooking water.