Place all of the ingredients for the brine together in a large pan and bring to the boil. Once it reaches the boil remove from the heat and leave to cool. Once it is cool place the brisket in the brine, making sure it is completely submerged. Refrigerate for a week.
For the braising liquor, fill a large pan with the water and add the vegetables and herbs. Bring to the boil and then reduce the heat to simmer. Add the brined beef and leave to cook for 3-4 hours until the meat is tender. Take the pan off the heat and leave to cool.
When the beef has cooled, dice it and then coarsely mince it along with the pork fat and pork belly in a meat mincer. Transfer to a large mixing bowl and thoroughly combine the beef and pork. Add the cornichons, capers, onions, mustard, pepper and Nam Pla fish sauce. Mix everything together and season as required.
Transfer the corned beef mixture into a terrine mould or metal frame lined with cling film. Ensure the top of the mould is fully covered with cling film, compress with a weight and place in the fridge to set overnight.
To serve, take the beef out of the fridge and remove from the mould. Evenly slice with a sharp knife and place a piece on each plate, brushing the upward facing side of the beef neatly with the wholegrain mustard. Distribute the chopped chives and mustard seeds over the top and add the fennel fronds, sliced gherkin, caper berries and cornichons to the plate. Serve immediately.
Recipe courtesy of www.greatbritishchefs.com