Photograph: Martin Poole
Preheat the oven to 220°C, fan 200°C, gas 7. Put the beetroot in a roasting tin and drizzle with the olive oil. Scatter over half the thyme leaves and season. Roast, covered with foil, for 45 minutes until tender. Set aside 40g of the cooked beetroot and finely shred it, for the garnish.
Put the butter in a pan and melt over a low heat. Add the roasted beetroot, along with any juices, and the remaining thyme leaves. Fry gently for a minute or two, then add the stock and bring to the boil. Simmer, partially covered, for 20 minutes.
Purée and season the soup, in batches if necessary, until velvety smooth. Toast the slices of baguette, then top with the sliced spring onion and crumbled feta, drizzle with a little olive oil and toast again for 1 minute. Serve the soup topped with the crème fraîche, shredded reserved beetroot and a few thyme leaves, with the toasts on the side.
Get ahead: make up to two days ahead. It can be frozen.
Kitchen secret: to save time, use 1kg ready-cooked beetroot (not in vinegar) and start the recipe at step 2.