Cook the beetroot in their skins in boiling salted water for about an hour or until tender. (If using different colours, cook them separately.) Drain and leave to cool; then rub off the skins (you may wish to wear gloves while doing this to keep your hands from staining).
Cut off the woody ends from the asparagus. Cook the spears in boiling salted water for 5-6 minutes or until tender. Drain and leave to cool.
Cut the beetroots into bite-sized pieces (a mixture of slices and wedges). Cut the asparagus spears in half.
Make the dressing by whisking the cider vinegar and rapeseed oil with salt and freshly ground black pepper to taste.
To serve, carefully arrange the beetroot and asparagus on plates with the sorrel leaves, then scatter the goats' cheese and pickled walnuts over the top. Drizzle over the dressing just before serving.