To make the mayo, just mix everything together.
Cut the fish into strips - they should be about the length of regular fish fingers, and half their width.
Sprinkle the flour and breadcrumbs onto separate plates and beat the eggs in a broad flat soup bowl (a soup plate just makes life easier). Dredge the haddock strips in the flour, then dip in the egg, then coat in the breadcrumbs. Set them on a clean plate or tray as they’re ready.
Heat about 4 tbs of oil in a large frying pan and cook the goujons until golden on both sides until deep gold and cooked through. They will need about 3 minutes each side. Cut into one of them with the point of a sharp knife to check that the fish flesh is white and no longer glassy looking.
While the goujons are cooking cut the base off the lettuce and discard. Shred the rest finely. Heat the butter and sauté the onions and lettuce until wilted. Boil the peas for 3 minutes then drain and add to the onions and lettuce. Season and add a good squeeze of lemon. Serve in a heated bowl with the fish and mayo.