It's not generally recommended that you make a fish stock with the bones from sea trout or salmon – they don't produce a very clear stock because they are oily fish. But it's such a shame to throw salmon and trout bones away, and I find that if you blanch the bones first and then make a stock as normal, you end up with a good stock which is ideal for a soup like this.
You could make this with a couple of sea trout or salmon tails, which are often better value than the prime cuts of salmon, or you could just ask your fishmonger for some salmon bones.
For the stock
Sea trout or salmon bones (see above), washed
1 onion, peeled and roughly chopped
2 sticks of celery, roughly chopped
1 bay leaf
A sprig of thyme
1tsp fennel seeds
1ltr fish stock
For the soup
A piece of sea trout or salmon fillet
50-60g samphire with the woody ends removed
3-4tbsp double cream
Put the sea trout bones into a saucepan, cover with cold water, bring to the boil and then drain and refresh under the cold tap.
Return the bones to the pan with the rest of the ingredients, bring to the boil and simmer gently for 30 minutes, then drain through a fine-meshed sieve into a clean pan.
Bring the stock back to a simmer and poach the sea trout or salmon fillet for 2-3 minutes, then remove the fish and leave to cool.
Meanwhile, melt the butter in a pan and stir in the flour; stir on a low heat for 10-15 seconds, then whisk the flour mixture into the simmering stock and continue simmering for another 15 minutes, giving it an occasional whisk along the way. Add the double cream and re-season if necessary.
Add the samphire and simmer for another minute. To serve, re-heat the sea trout or salmon for a minute or so in the soup, remove and break into chunks and arrange in the centre of warmed soup plates, then pour the soup and samphire over.