Step 1 Preheat oven to 300 degrees F (150 degrees C).
Step 2 Mix flour, sugar and butter until crumbly. Stir in chopped walnuts.
Step 3 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.
Step 4 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.
Step 5 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.