1. Preheat the oven to 180 C, gas mark 4. Wash the beetroot and trim the stalks to 1cm, but leave the skins and roots on. Wrap each wet beetroot individually in foil and place in a roasting tin. Cook for 1 hour 30 minutes until the beetroots are easily pierced to the centre with the tip of a knife.
2. Using a knife, slip off and discard the skins. Chop the flesh and place in a blender with the garlic, vinegar, thyme and butter. Season and whizz to a smooth purée. You can prepare in advance up to this stage and keep in the fridge.
3. Gently warm the purée through in a saucepan, stirring regularly. Tip into a bowl and swirl the soured cream into the top with a brief, cavalier flourish. Scatter with the extra thyme leaves and serve.