Soak the tarama in warm water a few minutes, then rinse and drain. If using bread slices, discard crusts, soak bread in water, then squeeze dry. Using an electric blender or electric mixer (or old-fashioned mortar and pestle), thoroughly blend the bread or potatoes and the tarama. Add the olive oil as slowly as possible, blending at medium speed, plus a little water if the mixture forms peaks. Add the grated onion, if desired, then whip at high speed for a few minutes. Note: The flavor may be mitigated to suit taste by adding more bread or potato; a too-salty taste may be adjusted by dropping in a few tablespoons of sour cream while blending. Leftover taramasalata can be stored in a covered jar in the refrigerator for 7 to 10 days.