Put 2 cups of raspberries in a blender with the sugar and lemon juice and puree until smooth. Strain through a mesh strainer into a small bowl, discarding the seeds once strained.
In a separate mixing bowl, beat together cream and yogurt with an electric mixer on high speed for several minutes until stiff peaks form. (To speed up this process, use a metal bowl and put it in the freezer about 10-20 minutes before beating the cream - this will drastically reduce the time needed to whip the cream.)
Stir in the raspberry mixture and then pour into the crust.
Top with the remaining cup of raspberries and freeze until firm (about 5-6 hours).
Drizzle with chocolate syrup if desired and set out for about 10-15 minutes prior to serving. Enjoy!