The trick here is to come prepared. Each time I buy a whole fish I keep the head and the bones in the freezer for a while. And when we eat nice fat grilled gamba's, with garlic, parsley and olive-oil I keep the heads.
When I think there is enough, i take the frozen fish bits from the freezer. Bring 2 liters of water to the boil and add the fish and gamba bits, with some onions, celery stalks, carrots, bay leaf, thyme and crushed peppercorns. An hour later I take everything out and strain the fishstock trough a sieve and reduce it till only a 1/2 liter is left. Which gets in the freezer.
The moment we expect guests I buy 12 big juicy Spanish mussels, a handful of vongole, some salmon and 4 juicy gamba's. Plus my secret ingredient: a jar of 'Soupe de Homard a la Bretagne'.
Readymade lobstersoup, yes. But which still need my concentrated fish stock from the freezer to become perfect. So I combine the two an heat them up slowly. I steam the mussels and vongole. Fry the gamba's and cut the fresh salmon.
The salmon cubes go raw in the plates and will be
slightly cooked when I pour over the soup.
The mussels and vongole follow as do the prawns.
Served with bread and a very garlicky rouille
(which comes for free with de Soupe).