Heat your oven to 220°C/Fan 200°C/Gas 7. Set a large roasting tin on the top shelf to heat up.
Trim and peel your beetroot and carrot. Cut into 2-3cm chunks. Tumble into the warm roasting tin. Drizzle over 1 tbsp oil and season well. Roast for 25 mins.
Meanwhile, halve, peel and thinly slice your onion. Thinly slice the celery. Add them to the beetroot and carrot. Roast for a further 10 mins or till everything is tender and a little coloured around the edges.
Halve, stone and thinly slice the plums. Pile into a little jar with one of the star anise. Cut a few thin slices from your chilli and add to the jar. Pour 3 tbsp balsamic vinegar to cover. Swirl everything together. Set aside to steep.
Tip the hot roasted veg into a blender. Blend with 500ml-1ltr boiling water and a good pinch of salt and pepper, trickling the water in slowly as you blend, till it’s the right consistency for you. Let the motor run for 1-2 mins to get a really smooth consistency. Blitz in a few slices of chilli and the remaining star anise, ground to a powder, if you have a pestle and mortar.
Simmer the soup for 5 mins to warm through. Serve the soup topped with the balsamic plums and drizzle of the juices from the jar.