Place the halved tomatoes in an ovenproof dish. Throw over the garlic and drizzle over half the olive oil. Roast them for 25-30 minutes at 190°C/375°F/Gas mark 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 purée, transfer the soup to a blender and process until completely smooth. Taste and adjust the seasoning if necessary. 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.