Wash the mussels and cockles.
Put them in a large pan over a high heat to open the shells, shaking from time to time.
Tip out of the pan and remove the mussels and cockles from their shells.
Filter and retain the juice.
Melt the finely chopped shallot in oil.
Season the filtered juice with olive oil and reduce by half.
Add the shellfish and prawns. Season then add crème fraîche and chives.
At the same time, cook the tagliatelle in salted boiling water, following the cooking time indicated on the packaging (5-6 min).
Season with olive oil again and pour over the seafood sauce.
Serve immediately.