Meanwhile, heat the oil and butter in a large (12-inch) saute pan over medium heat. Add the pancetta and cook for 5 minutes, until starting to brown. Stir in the leeks and cook over medium heat for 8 to 10 minutes, until the leeks are tender. Stir in the mushrooms, tarragon, sherry, 1 tablespoon salt and 11/2 teaspoons pepper and cook for 10 to 12 minutes, until most of the liquid evaporates, stirring occasionally. Off the heat, stir in the parsley.