(based on this recipe, with no hazelnuts: http://www.firstfruittable.com/cauliflower-soup/)
I'd try giving it a whiz with an immersion blender!
Agree, and I would do the blending shortly before serving (don't give that soup a chance to separate again).
Would adding cream/milk at this point and then reblending help at all?
By the way, I think the hazelnuts in the original recipe helped emulsify the soup and keep it together.
If you have some and no reason not to use them, try adding them.
Other possible emulsifyers, to hold the soup together - small amounts of egg yolk, mayo, prepared mustard