While the cake is baking prepare the red berry filling. In a medium saucepan combine 1 cup of strawberries, 1 cup of raspberries, lemon juice, water, sugar, and cornstarch. Bring to a boil, reduce heat, and simmer until the sauce starts to thicken but is still thin enough to pour about 3-5 minutes. Remove from the heat mash the berries and stir in the remaining 1/2 cup of strawberries and raspberries.
1 1/2 cups Diced strawberries, 1 1/2 cups Raspberries, 1/2 cup Granulated sugar, 1 tbsp Lemon juice, 1/4 cup Water, 2 tbsp Cornstarch