Preheat oven to 140C.
Drizzle the brisket with a little olive oil and season with sea salt and freshly ground black pepper. Preheat a large frying pan and sear the brisket to get some good colour on the outside.
Place in a snug-fitting, deep, ovenproof dish or pan. Cover with the beef stock, add the onion and garlic. Cover the dish with a lid or foil and cook slowly for 4-5 hours, until the beef is tender and giving.
Put the potatoes in a large saucepan, cover with salted water and bring to a boil. Simmer the potatoes until they’re tender and then drain. Leave the potatoes to steam in a colander for a few minutes to dry out.
In a small saucepan, heat the cream until boiling. Place the potatoes back in the large saucepan and pour over the warm cream. Mash the potatoes together with the cream until smooth, then stir in the butter in small chunks followed by the chopped sauerkraut. Cover the surface of the mash with a sheet of baking parchment whilst you prepare the kohlrabi.
Slice the kohlrabi as thinly as you can, using a mandolin if you have one to hand. Once you have sliced the kohlrabi into discs slice them again to form fine matchsticks. Place in a small bowl with the mustard, chopped dill, a drizzle of olive oil and pinch of sea salt. Stir to combine.
Once the beef is cooked, carve or pull apart into generous pieces. Serve with the sauerkraut mash, kohlrabi salad and dill pickles. Drizzle the brisket with a little of the cooking liquor.