Mix together flour, nuts, sugar, and butter. Spread in a 13 x 9 x 2" pan. Bake in moderate oven (350°) 20 minutes, stirring every 5 minutes. Remove from the oven and stir again. Remove crumbs from pan and cool. Sprinkle two-thirds of crumbs, about 1 1/3 c., back in the pan to cover the bottom.
Combine egg whites, berries, sugar, and lemon juice in large mixer bowl Beat at high speed until stiff peaks form, 10 to 15 minutes. Fold in whipped cream. Spread lightly over crumbs in pan. Sprinkle remaining crumbs on top. Freeze until firm. Cut in squares to serve. Yield: Makes 12 to 16 servings.