Place raspberries in a food processor and process until completely pureed. Pour the puree through a strainer over a small sauce pan and using a spatula, firmly press the puree through the strainer. Repeat until you extract as much puree as possible. Add lemon juice and sugar to raspberry puree and place on stove. On medium heat, add cornstarch and whisk constantly until puree comes to a boil and slightly thickened, keep whisking for about one minute and then remove from heat. Place in a bowl and allow to completely cool.