If the roe is fairly salty, soak the cod’s roe in water for 2 hours, then drain thoroughly and peel off the skin. If the roe is not too salty and the skins are soft enough, however, simply peel off without soaking first.
Using a fork, mash the cod’s roe in a bowl, and add the onion. Add a little of the olive oil, then transfer to a blender or food processor. Purée into a smooth paste.
Moisten the bread and gently squeeze out any excess water. Continue blending the cod’s roe mixture, alternately adding small bits of moistened bread, the remaining olive oil and the lemon juice. Blend until the taramasalata is smooth and cream in colour. Serve with pitta bread.