Preheat the oven to 350°F (175°C) and grease two 9-inch cake pans.
In a large mixing bowl, combine the white cake mix, all-purpose flour, granulated sugar, and salt.
Add the water, vegetable oil, vanilla extract, sour cream, and egg whites, and beat on low speed until just combined.
Increase the speed to high and beat for an additional 2 minutes.
Divide the cake batter evenly between the two prepared cake pans.
Bake the cakes for 30-35 minutes, or until a toothpick inserted into the center of the cakes comes out clean.
Allow the cakes to cool completely in the pans.
In a food processor or blender, puree the sliced strawberries until smooth.
In a large saucepan, whisk together the strawberry puree, granulated sugar, cornstarch, and water.
Cook the mixture over medium heat until it thickens, stirring constantly.
Allow the strawberry filling to cool completely.
In a mixing bowl, combine the all-purpose flour, brown sugar, melted butter, and chopped pecans, and stir until fully combined.
Divide the crunch topping evenly between the two cooled cakes.
Place one cake layer on a serving plate and spread the strawberry filling on top.
Place the second cake layer on top of the filling.
Sprinkle the remaining crunch topping over the top of the cake.
Chill the cake in the refrigerator for at least 30 minutes before serving.