Preheat the oven to 250°C / 475°F / Gas Mark 9.
Heat the oil in a pan and cook the spring onions for 1-2 mintues. Flake the fish into a bowl with the chopped herbs, seasoning, mashed potatoes and lemon juice. Add the cooked spring onions and lightly mix.
Shape the mixture into 24 fat cakes about 4cm across. Dip into flour, egg and then breadcrumbs. Chill for 15 minutes.
Melt the butter and brush all over the fishcakes. Cook for 20 minutes or until golden brown. Alternatively, shallow fry in oil.