Preheat the oven to 180C/350F/Gas 4.
Scrub the mussels, remove the beards and discard any mussels that remain open when tapped against the side of the sink.
Heat the olive oil in a large, heavy-based pan that has a lid. Add the onion and the parsley and cook gently until the onion is soft but not brown.
Add the mussels, salt and freshly ground black pepper and the white wine. Bring the wine to the boil, put the lid on and shake the mussels about. Leave the pan on the heat and cook for 3-5 minutes or until the mussels are open. Discard any that remain closed.
Strain the mussels and set aside, reserving the cooking liquor. Pour the liquor through a fine sieve into a clean jug to remove any grit.
For the sauce, melt the butter in a saucepan over a low heat. Gently cook the onion, carrot and celery until soft. Stir in the curry powder and continue cooking for a few minutes.
Add 150ml/5fl oz of the cooking liquor from the mussels, stir well and cook for another minute. Stir in the double cream, bring back to the boil then add the diced potatoes and cook for about five minutes or until the potatoes have softened.
Remove the mussels from their shells, add them to the cream mixture and warm them through. Finish the sauce with the chopped chives and season with salt and freshly ground black pepper and squeeze of lemon juice.
For the salmon, wash the fish and dry the skin thoroughly. With a sharp knife, cut diagonal slashes in the skin and season with salt and freshly ground black pepper.
Heat a non-stick ovenproof frying pan over a medium heat then add the vegetable oil. Cook the salmon, skin-side down, for 4-5 minutes or until the skin is crisp, then turn the fish over and transfer the pan to the oven for 3-4 minutes or until the fish is just cooked through.
To serve, place the curried mussels in a large bowl and arrange the salmon fillets on top.