Place the halved tomatoes in an ovenproof dish. Throw over the garlic and drizzle over half the olive oil.
Roast for 25-30 minutes in a fairly hot oven (190C/375F/Gas 5) until soft and pulpy. Rub through a sieve to remove the skin and pips.
Heat the remaining oil in a pan and sweat the onion for a few minutes until soft. Add the beetroot and the stock and bring to the boil. Season lightly with salt and freshly ground black pepper. Simmer gently for 7-10 minutes until the beetroot is tender.
Stir in the tomato purÃ©e, transfer the soup to a blender and process until completely smooth. Taste and adjust the seasoning if necessary
To serve cold, chill the soup in the fridge, then divide between six bowls. Using your fingers, crumble a little feta into each bowl. A sprinkling of grated raw beetroot makes a good garnish for the cold version
To serve hot, reheat the soup until thoroughly hot but not boiling. Divide between warm bowls and crumble over a little feta into each bowl. Serve with crusty bread.