In a medium saucepan, combine sugar, cornstarch, salt and lemon zest. Blend in water. Fill a 2 cup measuring cup with strawberries and blueberries, Add them to the mixture. Bring it all to a boil, stirring and smashing the berries deliberately (use a potato masher if you like). The blueberries are easier to smash then the strawberries. As they cook smash the strawberries more. Boil the mixture stirring constantly, until mixture is very thick, about a minute or so. Remove the glop/matrix from heat and stir in butter and lemon juice. Cool.