Meanwhile, place 2 pounds frozen mixed berries, 3/4 cup of the granulated sugar, and the remaining 1 1/2 tablespoons all-purpose flour in a large saucepan and stir to combine. Cover and cook over medium-low heat, stirring often, until the mixture is syrupy, thickened, and the berries break down but still maintain their shape, 6 to 8 minutes. Uncover and let cool completely, about 45 minutes.
