Add all ingredients to saucepan and cook over low heat until rhubarb is cooked, about 15-20 minutes. Allow to cool
Transfer to a blender and blend until it's completely smooth and is paler in colour
Place in the fridge to cool completely
Pour into an ice cream maker and freeze. Serve immediately or transfer to an airtight container and place in the freezer for 20-30 minutes to firm up
If you don't have an ice cream maker, pour mixture into an airtight container and place in the freezer. After 40 minutes remove from the freezer and give it a good stir with a fork. Repeat this process a few times until the mixture has hardened considerably. Serve