Elisabeth's Double Good Blueberry Pie

1. Mix the sugar and cornstarch in a saucepan.  Add the water and 2 cups of berries.  Cook over medium heat, stirring until the mixture is thick, clear, and boiling.  Remove from heat, stir in lemon juice, and cool.
2. Place remaining 2 cups of blueberries in the pie shell.  Top with cooked mixture.
3. Chill.  Serve with a sauce of half yogurt, half sour cream (if desired).