1) Chop the onion and celery.
2) Trim the greens off the cauliflower and slice into florets. Cut some of the florets in half.
3) In a large heavy pan, melt butter and then add onions, celery and cauliflower. Let this cook for 5-10 minutes over high heat until everything starts to soften.
4) After veggies have cooked for about 10 minutes and are soft, add the stock and milk and a small pinch of salt and pepper. Bring the soup to a simmer and then cook, covered, for about 25 minutes or until the cauliflower is extremely tender.
5) Take the soup off the heat and blend it with immersion blender (or regular blender if you do not have an immersion one).
6) To make croutons, take old bread and chop up a piece or two of it. Toss the bread with a drizzle of olive oil and a sprinkle of kosher salt. Toast at 350 degrees until they are very brown and crunchy.