Cook 1 cup raspberries in ¾ cup water for 5 minutes. Strain, retain liquid, and mix sugar and cornstarch into hot liquid and cook until clear, stirring often. Soften gelatin in 2 tablespoons cold water and add to sugar and cornstarch mixture. Let cool. Add 2 cups berries and lemon juice. Pour into baked pie shell and chill. Top with whipped cream and extra berries if desired.