Heat oven to 350°.
Cut butter into cake mix (dry) until crumbly.
Reserve 1 1/2 cups crumbly mixture; pat remaining mixture lightly in ungreased oblong pan., 13 X 9 X 2 inches, building up 1/2-inch edges.
Spread pie filling over mixture in pan to within 1/2 inch of edges.
Mix walnuts and reserved crumbly mixture; sprinkle over top.
Bake 45 to 50 minutes.