Mix flour, pecans, butter, and brown sugar together in a bowl until crumbly; spread onto a rimmed baking sheet.
Bake in the preheated oven, stirring often to break up crumbles, until lightly browned, about 10 minutes. Cool crumble to room temperature.
Sprinkle 1/2 of the crumble into a 9x13-inch baking dish.
Beat raspberries, white sugar, egg whites, and lemon juice together in a bowl using an electric mixer until smooth and creamy, about 10 minutes. Fold whipped topping into raspberry mixture; pour over crumble in the 9x13-inch dish. Sprinkle remaining crumble over raspberry mixture.
Freeze raspberry mixture until solid, at least 2 hours. Remove dish from freezer and let stand 10 minutes before cutting into squares.
