In a blender or food processor combine cannellini beans, beets, tahini, lemon juice, horseradish, garlic, and salt. Cover and blend or process until nearly smooth. With the motor running, add oil in a thin, steady stream through the feed tube.
Transfer beet mixture to a serving bowl. If desired, cover and chill for up to 24 hours. If desired, garnish with hard-cooked egg and parsley. If desired, serve with cucumber slices and/or celery sticks.