Begin by tearing the stale bread into small pieces and soaking it in a bowl of water for about 10 minutes until soft. Squeeze out excess water thoroughly so the bread is moist but not dripping.
In your food processor, combine the soaked bread, white fish roe, finely chopped onion, and lemon juice. Pulse a few times just to start mixing everything together.
With the processor running on low speed, drizzle the sunflower oil in a very slow, steady stream to emulsify and create a fluffy, creamy texture. This can take several minutes.
If the dip feels too thick, add the optional water a little at a time, blending after each addition, until you reach your desired consistency.
Taste and adjust the lemon juice or onion if desired. Transfer to a serving bowl, cover, and chill for at least an hour before serving to let the flavors meld.
