Set the oven at 200C/gas mark 6. Trim the beetroot but don’t peel them. Put them in a pan of water and bring them to the boil. Simmer for 25 minutes or until they will just take the point of a skewer.
Drain them and peel away the skin. It should come off easily. Put them in a roasting tin with the carrots, Jerusalem artichokes and parsnips halved or quartered, as you think fit, and the whole unpeeled onions.
Trickle a little olive oil over them. Pull the thyme leaves off their twigs and add to the vegetables along with the whole garlic cloves. Season lightly with salt and cover the roasting tin loosely with foil. Bake for 45 minutes, then remove the foil and cook for a further 20 minutes.
Make the dressing. Put the vinegar into a small bowl. Add a good pinch of sea salt and let it dissolve. Stir in the mustard and horseradish, then whisk in the honey and oils. When the roots are ready, toss gently with the dressing and serve at once.