Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking dish.

Line a rimmed baking sheet with aluminum foil. In a bowl, stir together the flour, brown sugar, walnuts, and melted butter until crumbly.

Spread the mixture onto the prepared baking sheet and toast in the oven for about 15 minutes, stirring occasionally, until the walnuts are fragrant.

Sprinkle 2/3 of the toasted walnut mixture evenly over the bottom of the greased baking dish to form the crust.

In a large bowl, whip the egg whites with the lemon juice until soft peaks form. Gradually add the white sugar and continue whipping until firm peaks form.

Fold the sliced strawberries gently into the egg white mixture.

In a separate bowl, whip the heavy cream until stiff but not grainy.

Fold the whipped cream into the strawberry mixture until smooth and fully combined.

Spoon the filling evenly over the crust and smooth the top.

Sprinkle the remaining 1/3 of the walnut mixture evenly over the top.

Freeze for 6 hours, or overnight, until fully set.