Slice 3/4 of the strawberries and place in a bowl.
Puree remaining strawberries in a food processor until smooth and transfer to a saucepan over medium heat. Add sugar, water, cornstarch, and lemon juice and bring to a boil, stirring often, about 5 minutes. Stir continuously until as thick as jam, about 5 minutes more. Pour mixture over sliced strawberries in the bowl and stir to combine filling.
Pour filling into pie shell and refrigerate until set, about 3 hours. Slice and serve with a dollop of whipped topping.