Soak the bread in water for 10 minutes. Squeeze it gently so it is still quite moist and put it in a food processor or blender. Skin the fresh roe if you are using it and add it, with the lemon juice and the onion, if used. If you are using salted tarama, add it as it is. Blend for 2 minutes until well amalgamated and then slowly add the oil, while the blades are running. Blend for 1-2 more minutes and then taste it. If you would prefer a milder taste, add a little more bread and oil; if the taramasalata is too stiff, 1-2 teaspoons of water will remedy the situation. It should be, not runny, but of a rather loose consistency. Spread on a platter and garnish with olives.