Mix butter, flour, brown sugar, and nuts lightly and put in a 9x13" pan.
Bake at 375° until golden brown (about 15 minutes), stirring occasionally.
Cool. Put half of mixture in the bottom of the 9x13" pan and reserve half for top of dessert.
Place (partly frozen) strawberries and egg whites, sugar, lemon juice, and vanilla in a large bowl. Use electric mixer and beat 20 minutes. Fold in whipped cream. Sprinkle remaining nut mixture on top. Freeze.
Take out of freezer and cut into squares to serve. Optionally garnish with fresh strawberries, blueberries, or raspberries.