Combine water, sugar, lemon juice and cornstarch in a large, heavy pan and slowly bring to a boil, stirring continuously to avoid lumping of cornstarch.
Add fruit and bring up to temperature, cooking for about five minutes, stirring continuously until thickened.
To bake a covered or uncovered pie, use a 9" pie crust. For uncovered, you may put dabs of butter on top of the fruit. Bake at 425° for 1/2 hour and then reduce temperature to 350° for another 1/2 hour.