To make the curing mixture for the salmon, place the salt, sugar and crushed white peppercorns in a bowl.
Add half the dill and stir to combine.
Use cling film to line a large, shallow, rectangular dish which fits the salmon comfortably.
Sprinkle a quarter of the curing mixture over the base of the dish and lay one of the salmon fillets on top, skin side down.
Sprinkle half of the curing mixture on top and cover with the other salmon fillet, skin side up.
Sprinkle the remaining curing mixture on top and wrap the salmon in cling film.
Weigh the salmon down with some weights or cans to help squeeze out any excess liquid or moisture. Place in the fridge for 2-3 days, turning the salmon over every 6 hours or so, replacing the weights after turning.
After 2-3 days, unwrap the salmon. Rinse the curing mixture off the gravadlax using cold water and pat fish dry with kitchen paper.
Lay a large piece of cling film on the work surface and place one of the fillets on top, skin side down. Cover with the remaining fresh dill and place the other salmon fillet on top, skin side up. Wrap tightly in cling film and chill for six hours.
Meanwhile, to make the sauce, place the mustard, sugar, vinegar and egg yolk in a large bowl and whisk to combine.
Add the oil drop by drop to begin with, then in a steady stream, whisking constantly, until the sauce becomes thick and smooth.
Stir in the dill and season to taste.
To serve, cut the gravadlax into thin slices using a sharp knife, leaving the skin behind. Gravadlax is traditionally served sliced thicker than smoked salmon.
Place three or flour slices on each serving plate and add a spoonful of the sauce to the side.
Serve at once with rye bread.