An immersion stick blender will work equally well. Or make the puree on the stovetop. Add the raspberries into a saucepan and add 2 or so tablespoons of lemon juice. Heat the raspberries on low until they start to break down. Gently mash them with a fork to release their juices.  Once all the raspberries are broken up and mashed, remove the pan from the heat and pass the sauce through a fine-mesh sieve. Carry on as per the recipe method. Just let the purée come to room temperature before adding to the ice cream base.