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.
This recipe contains raw eggs. We recommend that pregnant women, young children, the elderly, and the infirm do not consume raw eggs. Learn more about egg safety from our article, Making Your Eggs Safe.