Put the roasted cauliflower, broth, celery, and scallions in the pot on the stove and turn the heat to high. Bring to a boil (you’ll know it’s boiling because you’ll see little bubbles breaking at the surface). Add the spinach and cook for 1 minute. Turn off the heat and set aside to cool for 10 minutes.
Remove half the solids, using the slotted spoon, and put them in the blender. Do not fill more than halfway. Put the top on but remove the little cap in the center. Cover the hole loosely with the dish towel. Turn the blender to the lowest speed and increase the speed as the soup purees. Blend until smooth. Add the remaining solids and blend again.
Add the yogurt or milk and blend again, then pour the soup back into the pot and taste it. Does it need a pinch of salt? If so, add it and taste again. Reheat gently over low heat, then serve right away or cover and refrigerate overnight.