Boiled beef also goes remarkably well with Aïoli or Green Sauce, but if using these, do not include the dumplings or the pickled walnuts. To salt the briskets yourself, make a brine and leave them for 12 days, or you can buy salted brisket from the butcher, in which case make sure it’s not rolled in.
Place your beef into a pot (remember it has to be big enough to accommodate the vegetables as well), cover with water, and add your herb bundle, celery, bay leaves and peppercorns. Bring up to the boil, skim, and reduce to a very gentle simmer, with barely signs of movement in the water, for approximately 4 hours. Prod with a knife to check how the meat feels—it should be giving, but not collapsing!
After the meat has been simmering for 2 hours put in the onions, after 2½ hours the carrots, and after 3 hours the leeks. Keep an eye on your vegetables so they do not overcook—you can always remove them. However, this is a dish that demands well-cooked vegetables, no al dente here. When everything is ready, remove the meat and vegetables to a serving dish and keep them warm with a splash of broth to moisten. Make the dumplings as follows.
Bring the broth to a rolling simmer. Meanwhile mix the ingredients together, adding some cold water: you are looking for quite a sticky dough. Shape into walnut-sized balls and drop into your simmering broth—they should take about 10 minutes to cook and should be like little suet clouds. Serve all together with pickled walnuts and Horseradish Sauce.