I found this in an older cookbook and the pie is quite good.
Prepare prebaked pie shell and set to cool while preparing filling. Mix together ¾ cup sugar (more if berries are tart), ¾ cup berries, water and cornstarch to pan. Stir frequently over medium heat until thickened. Add remaining berries and cook until they start to break down ever so slightly and the mixture is blueish. Remove from heat. Add lemon juice then taste for sugar and add more if necessary. Pour into pie shell, refrigerate 3 hours (better overnight) until firm. Serve with whipped cream.