Remove crust from white bread and put it in a sieve. Soak it with water and let sit. Press out the water leaving fully soaked bread mush. Using a large mortar and pestle or food processor, mix together the Tarama and onion until well combined. Add the bread and mix together. Do not over mix. Add the juice of fresh lemon and combine. Slowly incorporate the olive oil, again, just mixing until combined. Serve with fresh slices of bread.