Cook the peas in simmering water for 10 minutes, then drain.
Mash with fork – this gives a better texture than pureeing it in the food processor – with the butter and seasoning. Add the vinegar and mint sauce – they give it a really good tang.
You can get this ready then reheat it at the last minute.
Cut the fish into fish-finger sized pieces, rubbing your hand over the surface to check for any little bones that might still be there. Season each piece.
Put the egg, flour and breadcrumbs into three plates. Dip each piece of fish in the flour, then in the egg, then in the breadcrumbs. Set them on a baking sheet where they can lie, ready to cook.
Heat the oil in a frying pan over a medium heat until a breadcrumb sizzles and turns golden-brown when dropped into it.
Put the fish pieces into the pan, in batches if you have to, and fry for 2-3 minutes each side, or until they are crispy and golden and cooked through.
Remove them as they are cooked with a slotted spoon onto a double layer of kitchen paper.
Turn over to blot up the excess oil and serve with the warm pea puree (and some oven-cooked potato wedges).