Mix corn starch and sugar in a medium saucepan.
Mash about 1 cup of berries. (You will need more than 1 cup of berries to make a cup of mashed berries. I usually start with a cup of whole berries and then add as needed to make a cup of mashed berries.) Add about 1/4 cup water to make 1-1/4 cup total.
Add the mashed berries to sugar and corn starch and cook until the mixture starts to thicken, about 8 to 10 minutes. Add lemon juice and continue to cook to thicken, another 3 to 5 minutes.
Place remaining raspberries in the pie crust, spreading them evenly over the bottom.
Pour the cooked berry mixture over the uncooked berries. Refrigerate at least two hours.
Serve pie with whipped cream.