Mix flour, sugar and butter until crumbly. Stir in chopped walnuts.
Press two-thirds of the mixture in the bottom of a 9x13-inch baking dish; the remaining crumble will be used for the topping. Bake in preheated oven for 20 minutes. Cool crust completely.
In large deep bowl, stir together egg whites, sugar (see Cook's Note), strawberries, and lemon juice. Whip with electric mixer at high speed for 10 minutes.
In another mixing bowl, whip whipping cream until stiff and fold into strawberry mixture. Spread over cooled crust. Sprinkle with remaining crumb mixture. Freeze for 6 hours or overnight before serving.
