Heat your oven to 200°C/Fan 180°C/Gas 6. Set a large roasting tin on the top shelf to heat up.
Peel the beetroot. Cut into 3cm chunks. Tumble into the warmed tray with 1 tbsp oil. Season well with salt and pepper. Roast for 20 mins.
While the beetroot cooks, halve, peel and chop the onion. Add to the beetroot. Roast for another 10 mins.
Chop the chilli, scraping out the seeds and white bits. Put the beetroot, onion and chilli in a blender with the stock cube, 1 tbsp garam masala, the coconut milk and 500ml warm water. Blitz till smooth. Or add everything to the roasting tin and blitz with a hand-held blender.
If your roasting tin is OK to use on the hob, pour the soup into that and warm through for a few mins. Alternatively, pour into a pan and heat through. Serve in warm bowls.