Place the beef in a flameproof casserole dish. Add enough water to cover the meat very generously. Bring to the boil over a high heat, skimming the surface as necessary to remove all the grey foam.
As soon as the liquid comes to the boil, reduce the heat to very low. Tie the thyme, parsley, bay leaf, garlic and peppercorns in a little muslin bag and add to the pan with the leek and celery. Half cover the pan with a lid and simmer gently, skimming the surface when necessary, for 1 3/4–2 hours or until the beef is very tender when pierced with the tip of a sharp knife.
Meanwhile, make the beetroot relish. Place the beetroot in a bowl with the spring onions and parsley. Season with salt and pepper to taste and gently stir together. Cover and refrigerate until needed.
Preheat the oven to low. When the meat is tender, use two large spoons to transfer it to an ovenproof dish. Spoon over enough of the cooking liquid to cover the meat, then tightly cover the dish with foil. Place in the oven to keep warm.
Remove the muslin bag from the casserole dish and discard. Add the potatoes and shallots to the dish, increase the heat and cook for 5 minutes. Add the turnips and carrots and simmer for a further 15 minutes or until all the vegetables are tender. Using a slotted spoon, transfer all the vegetables to the dish with the meat.
Add the cabbage to the broth in the casserole dish and simmer for about 3 minutes or until tender. Remove from the dish with a slotted spoon and add to the other vegetables.
To serve, slice the beef against the grain and place the slices in soup plates or plates with rims. Top with a selection of vegetables and spoon over some of the broth. Sprinkle with parsley and serve with the relish.