First make the Café de Paris butter. Combine all the ingredients, except the butter, in a large bowl. Set aside for 4 hours.
Beat the butter until fluffy, then mix with the other ingredients. Transfer to a sheet of cling film and shape into a neat log about 2.5cm/1in in diameter. Wrap in the cling film and chill. (This will make more butter than you need for this recipe, but it can be kept in the freezer for up to 6 months; it is good on steaks, chicken and fish.)
For the salmon, heat the wine with the shallots, garlic and thyme in a large saucepan over a low heat for 3 minutes. Turn the heat up and add the mussels and cockles or clams. Cover with a lid and give the pan a good shake, then cook for about 3 minutes or until all the shells have just opened. Discard any that do not open.
Tip into a sieve or colander set over a bowl so you can retain the juices. When cool enough to handle, remove the mussels and cockles from their shells and set aside; retain a few in their shells for garnish.
Return the juices to the pan and reduce over a high heat until you have 4 tablespoons left. Keep warm.
Heat a large frying pan until very hot. Add 1 tablespoon of the Café de Paris butter and the olive oil. Season the salmon with salt and freshly ground black pepper and place in the pan. Cook for 2-3 minutes on each side, depending on the thickness. Remove and keep warm.
Pour the shellfish juices into the hot fish pan and stir to combine with the pan juices. Add 2 tablespoons of the Café de Paris butter, the unsalted butter and the reduced passata and cook until emulsified. Check the seasoning. Add the shelled mussels and cockles, shrimps and scallops and warm through.
To serve, place the salmon on four warmed plates and spoon the shellfish butter over the fish. Garnish with the mussels and cockles in shell and the steamed potatoes.