Heat the oven to 220°C (425°F, gas mark 7). Cut the beetroot, carrots, parsnips and potatoes into large chunks and place them in a roasting tin, keeping the beetroot in one corner of the tin to prevent it from staining the other vegetables.
Slice off the top of the head of garlic and discard, then add the garlic to the vegetables. Scatter the rosemary and thyme over them and pour on the oil and 3 tablespoons of cold water. Roast for 40 minutes, stirring occasionally to prevent the vegetables from burning.
Remove the garlic and set it aside to cool a little, and continue roasting the vegetables for a further 20-35 minutes until they are tender.
Meanwhile, tear the bread into small pieces and soak it in the milk for 10 minutes. Squeeze the garlic cloves out of their skins, mash them well and beat them into the soaked bread to make a sauce. Stir in the horseradish and season to taste. Serve the roasted vegetables with the sauce, garnished with a sprig of fresh thyme.
Variation: if you cannot find raw beetroot, use cooked vacuum-packed beetroot, adding them to the roasting tin when you remove the garlic.