Trim and roughly chop the celery. Peel and roughly chop the carrots, then peel and roughly chop the onions.
Season the beef well and sprinkle with mustard powder. Pour 1 tablespoon of oil into a large pan over a medium-high heat.
Place the beef in the pan and sear for about 10 minutes, using tongs to turn it so that it’s cooked all over. Cover the beef with water.
Tie up the thyme and bay leaves into a bouquet garni and add to the pan along with the peppercorns. Add the chopped celery, carrots and onion. Bring to the boil, cover, reduce the heat and simmer for 4 hours.
Cut the red cabbage into wedges, trim the leafy vegetables, and slice the gherkins lengthways. Trim and peel the shallots.
After 3 hours 30 minutes, add the potatoes, baby carrots, baby turnips and shallots. Cook for a further 20 minutes. Add the cabbage and leafy vegetables for the last 10 minutes.
When the meat is tender, remove from the broth with tongs and slice or shred with a carving knife.
Divide the broth between bowls and serve the brisket and vegetables on a platter along with the sliced gherkins.