In a small bowl, combine flour and brown sugar, cut in butter until crumbly. Stir in pecans. Press into an ungreased 9x13 baking pan. Bake at 350 degrees for 14-16 minutes or until lightly browned. Cool on a wire rack.
Crumble the baked pecan mixture, set aside 1/2 to 1 cup for topping. Sprinkle the remaining into the rest of the pan and press into place.
In a large mixing bowl, mix the smoothie mix and lemon juice until blended. Fold in whipped cream. Spread evenly into dish. Sprinkle with reserved pecan mixture. Cover and freeze for 8 hours or overnight.