In a bowl, combine flour, walnuts, brown sugar, and salt. Cut in butter until it resembles course crumbs. Set 1/2 aside. Press the other half inside bottom of a greased 9 x 13 x 2 inch baking pan. Bake at 350 degrees for 10 to 15 minutes or until golden brown.
In a mixing bowl, beat filling ingredients until smooth. Pour over baked crust and sprinkle remaining crumb mixture over the top.
Bake at 350 degrees for 20 to 25 minutes or until knife comes out clean. Cool completely, store in refrigerator.