Heat the oil in a large pan and gently cook the onion and garlic for 10 mins until soft. Add the beetroot and vinegar to the pan and cook down until sticky. Take the pan off the heat, then stir in the crème fraîche. Blitz with a hand blender, or tip into a food processor and blitz to a purée. Stir in the grated horseradish and serve.
The purée can be made up to a day ahead and heated in a pan or in