When you have ripe summer berries, making a pie filling is simply a matter of tossing the berries with sugar and a thickening agent. Pastry chef Carolyn Weil shows how to make a fillng and keep the berries bright and intact.When you have ripe summer berries, making a pie filling is simply a matter of tossing the berries with sugar and a thickening agent. Pastry chef Carolyn Weil shows how to make a fillng and keep the berries bright and intact.