Preheat oven to 425 degrees. In a 12-inch saute pan cook the shallots in butter over moderately low heat, stirring, until softened. Add sliced mushrooms, Worcestershire sauce, and salt and pepper to taste. Cook the mixture over moderate heat, stirring occasionally, until the liquid from the mushrooms has evaporated. Add vinegar and Sherry and boil until all liquid is evaporated. Transfer to a bowl and cool. Whisk in cream, egg, and salt and pepper to taste. Filling may be prepared up to 2 days in advanced and chilled, covered.
On a lightly floured surface roll out pastry into a 16 by 10-inch rectangle and fit it into a 13 3/4 by 4-inch tart pan with a removable fluted rim or a 9-inch round tart pan. Roll a rolling pin over the pastry to trim it. Lightly prick the bottom of the shell with a fork and spread the filling evenly. Bake for 20 to 25 minutes or until pastry is golden and the filling is set. Cool to room temperature before serving and remove from pan. Garnish with fresh chives.