Mix all of the ingredients except the xanthan gum together in a bowl. Cover and let this sit for 30 minutes.
Add everything from the bowl to a pan with a lid and bring to a boil. Reduce to the lowest setting, put the lid on and cook for about 10 minutes or until the rhubarb is completely soft.
Remove the lemon zest, star anise and add to the blender. Start on low speed until it catches and then increase to high speed for about 1 minute. You may need to use the feed to blend it properly.
Once the texture is smooth, add the Xanthan gum, a little squeeze of lemon juice and continue blending.
Keep in a mason jar or lid with container for up to 5 days in the fridge. Enjoy!
Notes
If the pan gets too hot and the rhubarb sticks to the bottom of the pan, you may need to add more water.