Fish loves pasta. A twirl of fettucine with steamed lobster or sole is much more in harmony than potatoes of any sort. This recipe makes use of that affinity to produce a dish from the less expensive, though more fiddly, shellfish like cockles. If you have grander beasts to add, like langoustine or clams, so much the better. The key to the flavour is the molluscs, and sauce made without them won’t taste as good.