In a large bowl, combine the flour, walnuts, brown sugar and salt; cut in butter until the mixture resembles coarse crumbs. Set half aside; press remaining crumb mixture onto the bottom of a greased 13x9-in. baking pan. Bake at 350° for 10-15 minutes or until lightly browned. , In a large bowl, beat filling ingredients until smooth; pour over crust. Sprinkle with reserved crumb mixture. , Bake at 350° for 20-25 minutes or until a knife inserted in the center comes out clean. Cool on a wire rack for 1 hour. Store in the refrigerator.