450g/1lb mixed seafood, such as large prawns, peeled and their intestinal tracts removed, smoked haddock, cod, salmon, trimmed and cut into 2cm/¾in dice, or monkfish fillet, trimmed and cut into 3cm/1¼in strips
140g/5oz live mussels, washed and debearded (discard any that are open or do not close when tapped)
Slice the soda farls in half horizontally and then cut into rounds with a 10cm/4in cutter. Place on a tray ready for toasting. Preheat the grill to high.
Finely chop the bottom 4cm/1½in off the spring onions. Place in a large frying saucepan over a moderate heat with one tablespoon of the butter, cover and sweat for one minute.
Cut the remainder of the spring onions into 4cm/1½in pieces and set aside.
Add all the prawns, fish and mussels to the frying pan and turn the heat up to high. Fry for about a minute and then add the wine, the remaining spring onions and a little salt. Cover and cook just until the mussels are open (discard any that do not open) and then add the tomatoes, chives and the cream, if using, and the remaining butter. Shake the saucepan gently to incorporate all the butter into the sauce.
Toast the farls and place one in the centre of four warm plates. Spoon the fish and seafood generously over with some of the sauce. Garnish with a few sprigs of chervil and serve at once.