Preheat the oven to 200C/390F/Gas 6.
For the winter vegetables, peel the parsnip and chop each one into six lengthways. Trim out the core with a sharp knife.
Peel the carrots and chop each one into six lengthways. Peel the beetroot and chop into quarters.
In a large pan, heat the olive oil, then add the parsnips and carrots and fry for two minutes until the vegetables begin to colour.
Add salt and freshly ground black pepper, to taste, along with the butter, garlic and thyme. Fry for a further 5-6 minutes, or until the vegetables are tender all the way through. Add the beetroot, honey and vinegar to the pan and warm the beetroot through. Set aside, keeping the vegetables warm.
For the walnut paste, place the walnuts on a baking tray and roast them in the oven for five minutes. Remove the walnuts from the oven and rub them with a clean cloth to remove the skins. In a pestle and mortar, grind the peeled walnuts together with the garlic and olive oil. Season, to taste, with salt, freshly ground black pepper and lemon juice.
For the horseradish dressing, mix together the crème fraîche, mustard, grated horseradish and lemon juice and season, to taste, with salt and freshly ground black pepper.
To serve, spread the walnut paste on two plates and arrange the vegetables on top. Garnish with watercress and drizzle the horseradish dressing over.