Preheat oven to 350°. Mix flour, sugar and brown sugar; cut in butter until crumbly. Stir in walnuts. Reserve 1 cup mixture for topping; press remaining mixture onto bottom and halfway up sides of a 13x9-in. baking dish., In a large bowl, beat eggs, pumpkin, milk, cinnamon, salt and allspice just until smooth. Pour into crust; sprinkle with reserved topping., Bake until golden brown, 50-55 minutes. Cool slightly on a wire rack. Serve warm, or refrigerate and serve cold. If desired, dust with confectioners' sugar. Store leftovers in the refrigerator.