Put the cod fillet, cut from the thick end of the fish, into a large, shallow pan with the juice of two lemons, butter and half the tarragon, chopped. Add a bay leaf and black peppercorns, then bring to the boil, lower the heat, cover with a lid and simmer for about 10 minutes until the fish is opaque.
Remove the fish with a fish slice and keep it warm. Add the remaining tarragon, chopped, the capers and the crème fraîche.