Photograph: Jan Baldwin
Preheat the oven to 200°C, fan 180°C, gas 6. Put the beetroot in a roasting tin, drizzle with the oil and roast for 1 hour or until tender.
Remove from the oven and allow to cool slightly. Wearing rubber gloves, rub the beetroot to remove the skin, then discard it. Roughly chop the beetroot and set aside.
Melt the butter in a pan, add the onion and garlic and cook for 5 minutes until soft and beginning to colour. Add the vinegar; simmer for 3 minutes. Add the stock and beetroot and bring to the boil. Simmer for 5 minutes.
Meanwhile, toast the zest and mustard seeds in a dry frying pan for 2-3 minutes. Transfer to a plate.
Remove the soup from the heat and season. Blitz in a blender until smooth. Ladle the soup into mini mugs or bowls and top with the zest and mustard seeds. Drizzle with a little olive oil.
Get ahead: make the soup up to 2 days ahead; chill. It can also be frozen.