Place the sugar and salt into a bowl, then lightly crush the peppercorns and coriander seeds in a pestle and mortar and add them to the bowl. Stir in half of the dill.
Scatter one-third of the mixture into a large, deep-sided roasting tray. Lay the salmon fillet on top and cover it with the remaining sugar-salt mixture, pressing it gently into the fish.
Cover the roasting tray with cling film. Place the tray in the fridge for 24 hours to allow the salmon to cure.
Rinse the cured salmon under cold running water. Pat dry and place onto a chopping board.
Brush the top of the salmon with the Dijon mustard, then cover with the remaining chopped dill. Set aside.
Crush the juniper berries in a pestle and mortar, then place them in a frying pan with both types of mustard seeds, the sugar, red wine vinegar and a pinch of salt. Heat the mixture until bubbling, then simmer for 3-4 minutes, or until the seeds have absorbed some of the vinegar.
Remove the pan from the heat and stir in 50ml/2fl oz of the Champagne.
Transfer the mustard mixture to a food processor and blend for 3-4 minutes to a fine purée, adding the remaining Champagne halfway through blending. Season, to taste, with salt and black pepper.
To serve, carve the gravadlax into very thin slices using a sharp knife. Serve with the Champagne mustard alongside and pumpernickel bread, if desired.