I made vyprazeny karfiol (kvetak, since we’re now in the Czech Republic) the other day, and I saved back the water I used to steam the cauliflower plus all the bits that were to small to fry, along with one or two wobbly florets. We also had some leftover steamed potatoes from that meal, so the next day I made cauliflower soup.
Simply sauté a bit of onion in oil in the pot, then dump in the water, cauliflower, and potatoes. Bash a clove or two of garlic, and then toss that into the pot as well. Season with salt and pepper, simmer until the garlic is soft, then puree it all together. Finally, add in a bit of milk to make it creamier and check the seasonings. I topped it with chopped parsley, but you could also put in croûtons if you had them.
This was a hit, and I love the something from nothing aspect of it. The soup made enough for two servings, so I got a light lunch out of it as well.