Boil the potatoes in their skins in plenty of boiling salted water until they are tender. Drain and leave until cool enough to handle, then peel.
Skin the cod’s roe and put in a food processor with the onion. Puree until you have a smooth paste.
Cut the crusts off the bread and soak the slices in the milk, then squeeze out the liquid. Add the bread to the food processor and continue to puree. Then add the potatoes and puree again.
Gradually pour in the olive oil in a slow steady stream- not too fast or the mixture may split. Add the lemon juice, a little at a time until it is to your taste. Season with salt and pepper and serve with hot pitta bread.