Heat the butter in a saucepan, add the white sliced spring onions and the potato, toss in the butter then cover and fry gently for 10 minutes, stirring occasionally until only just beginning to colour.
Pour in the stock, add the bay leaf then lay the fish fillets on top and season with salt and pepper. Bring to the boil then cover and simmer gently for 20 minutes until the potatoes are tender. Lift the fish out of the soup with a slotted spoon, put on a plate and peel away the skin. Flake into pieces, carefully checking for any bones.
Return the fish to the saucepan, add the green spring onions tops, the frozen sweetcorn, defrosted shellfish and milk. Bring back to the boil then cover and simmer for 5 minutes until the seafood is reheated. Discard the bay leaf. Pour in the cream, add the parsley then taste and adjust the seasoning if needed. Bring back to the boil then ladle into hollowed out bread rolls. Scoop out the soup with a small spoon then eat the bread last when steeped in all the fishy flavours.