Combine the sugar, salt, crushed peppercorns, coriander seeds, and half the dill in a bowl.
Scatter a third of the spice mixture into a roasting tray and place the salmon on top.
Apply the remaining spice mixture to the salmon and cover the tray with cling film.
Cure the salmon in the fridge for 24 hours.
Rinse the salmon under cold water and pat dry.
Coat the salmon with Dijon mustard and the remaining dill.
Simmer crushed juniper, mustard seeds, sugar, and vinegar in a pan for 4 minutes.
Remove from heat and stir in 50ml of champagne.
Blend the mustard mixture in a food processor, adding the remaining champagne to create a fine purée.
Slice the salmon thinly and serve with the champagne mustard.