1 lemon, cut into wedges
2 white onions, finely sliced
2 shallots, finely chopped
1 bunch tarragon, leaves picked and chopped, few leaves reserved for garnish
4 salted anchovy fillets, chopped
2 bay leaves
salt and pepper
50ml/2fl oz olive oil
200g/7oz unsalted butter, softened
1 plaice (at least 1kg/2lb 4oz), gutted
200ml/7fl oz cider