1Heat 1 cup raspberries and 1/2 cup water in a saucepan over medium heat; cook and stir until raspberries soften, about 5 minutes. Strain raspberries into a bowl through a fine mesh sieve; discard seeds. Return mashed berries to the saucepan.
2Stir cornstarch and 1/4 cup cold water in a bowl until dissolved and stir into mashed berries; add sugar.
3Heat raspberry mixture over medium heat, stirring constantly, until thickened, about 5 minutes. Stir in lemon juice. Allow raspberry sauce to cool to room temperature.
4Line the prepared pie crust with remaining 3 cups raspberries. Pour raspberry sauce over berries and chill until set. Serve garnished with whipped cream and lemon zest.