Is there a substantial difference in using fresh vs frozen broccoli when making Broccoli Cheese Soup? And if I do make the substitution, should I thaw it first or put it in frozen?
If you use frozen you will not have the option of having a thick cheese soup with more crisp broccoli; i.e. if you were considering adding the broccoli toward the end after blanching it for a more toothsome floret, this would only be fruitful with fresh.
I am unsure about the blogspot link in Sebiddy's post, but my experience with frozen broccoli is that it gets the job done but not very well. As a point of logistics, using frozen without first thawing will net you a hearty release of water from the broccoli if you don't thaw first and may cause your mornay some heart-ache if timed poorly.
I can't imagine there would be a great difference as it's a soup and if you're worried about them becoming mushy then in broccoli cheese soup it should not make much of a difference as they're going to be blitzed anyway. If you were using them though you can keep them from going mushy and keep them green by adding them frozen to a hot pan with just a little water in, stirring often, more here.
Hope this helps!