Make the rhubarb ahead of time and store it in the fridge until you need it. Add other fruit if you like: it's a good way to use up over-ripe pears or soft apples. No extra sugar is necessary. If you're not a fan of star anise, add a cinnamon stick instead, or 1/2 tsp ground cinnamon, or 1 tsp vanilla bean paste.
The rolled oats can be left to soak in water with the spices overnight to save time in the morning if you like.