At least 2 days – but ideally up to a week – before you would like to serve the salt beef, make a start on the brine. Place all the brine ingredients in a large pan on a medium heat. Pour in 2 litres of cold water and bring to boil, then reduce to a simmer for 5 minutes until the salt has dissolved. Set aside and allow to cool completely. Meanwhile, remove the sinew and thick layer of fat from the meat, leaving a little behind for bonus flavour. Pierce the meat with a small sharp knife or skewer all over on both sides and place in a non-metallic sealable container. Pour the cooled brine over the brisket, then seal and store in the fridge for 5 to 7 days, turning the brisket over each day. When the brining time is up, remove the brisket from the container and soak in cold water for 10 minutes. Rinse the brisket well, then place in a large saucepan and cover with fresh cold water, until the beef is submerged. Peel and quarter the onions, then add to the pan with the bay and peppercorns. Bring to the boil, then reduce to a low heat and simmer for 2½ hours to 3 hours, or until the brisket is tender. Meanwhile, mix all the mustard ingredients together in a small bowl and season with a good pinch of sea salt and black pepper. Loosen with a drop more vinegar, if it looks a little thick. Slice up the gherkins and place them in a small bowl. Dress them with a couple of tablespoons of their pickling liquor and pick over the dill fronds. Finely slice the chillies – deseed if you prefer – then add to the bowl and mix together. Carefully remove the brisket to a board and thinly slice off what you need. (You can keep the rest of the meat in the liquor to stop it drying out.) To serve, split your bagels and spread a dollop of mustard on the base. Load up the pickles and pile on slices of the tender salt beef. Top with the bagel lid and tuck in!