You've got to get organised if you want to serve this when your friends are round next...it will take you at least 2 days to get the perfect salt beef! Don't worry though, you'll actually only need about 20 minutes work, then let the ingredients do all the work to make the most incredible beef!
put all the ingredients for the brine except for the beef into a saucepan and bring to the boil to simmer for a few minutes, then leave to cool.
place the beef and the cooled liquid into a water-tight container, make sure the beef is fully submerged and leave in the fridge to soak for at least 2 days... up to a week is good.
rinse the beef and soak in clean, cold water for 2 hours, then rinse again. put the beef and chopped vegetables into a pan and fill with water until it covers the beef.
simmer very gently for about 3 hours until the beef is tender, then drain and slice.
slice the bagels into halves. mix the mustard and mayonnaise together and spread on the bagels.
cram the mustard bagels with plenty of salt beef and sliced gherkins. serves 6