1. If making the salt beef from scratch, put the sugar and salt in a pan with 1.5 litres water. Heat until the sugar has dissolved, then cool completely. Pierce the meat all over with a fine skewer, then put in a large, sterilised, plastic bowl (or a large, double-lined freezer bag) and cover with the cooled brine. Leave in the fridge for 5 days (in a roasting tin, if using a freezer bag), turning every day.
2. Preheat the oven to 150˚C, gas mark 2. Take the beef out of the brine and soak it for 20 minutes in fresh, cold water, then rinse thoroughly. Put in a large roasting tin with the vegetables, spices, garlic, bay leaves and parsley stalks; add cold water until covered. Cover with foil and cook for 3-4 hours, turning halfway, until the meat is very tender. Drain the beef and discard the solids. (If using the ready-prepared salt beef, cook according to pack instructions.)
3. Gently shred the salt beef with two forks. If you’ve made your own, set aside 400g for the sandwiches and keep the remainder in the fridge for up to 5 days.
4. Butter 2 slices of bread and sandwich together with a slice of cheese. Heat a griddle pan until hot, then griddle on both sides, until the cheese has melted. Meanwhile, mix together all of the sweet mustard ingredients with a pinch of salt. Open up the sandwich, put the pickle on top of the cheese and drizzle over the mustard sauce. Pile on 100g of warm salt beef. Top with the other slice of toast and repeat with the remaining ingredients.