Preheat an oven to 230 C / Gas 8. Prick the potato in several places with a fork and place on a baking tray.
Bake the potato in the preheated oven until easily pierced with a fork, 50 minutes to 1 hour. Cool completely in fridge. Peel and chop.
Place the bread slices in a shallow dish; pour the milk over the bread to cover completely. Allow to soak a few minutes before squeezing as much moisture as possible from the bread, discarding the milk.
Blend the bread, potato, fish roe, onion, lemon juice and olive oil together in a food processor until fluffy, about 1 minute. Add more olive oil if needed to reach a desired consistency; chill. Garnish with Greek olives to serve.