Combine florets and oil in a large bowl; toss to coat. Add thyme and salt; toss well. Arrange florets on a jelly-roll pan coated with cooking spray. Bake at 400° for 1 hour or until golden brown, stirring after 30 minutes.
Combine florets, 2 cups broth, and milk in a blender. Process 3 minutes or until smooth. Pour pureed mixture into a large saucepan. Add remaining 2 cups broth and pepper. Cook over medium heat until thoroughly heated. Garnish with fresh thyme sprigs, if desired.
I only had one head of cauliflower, so halved the recipe. I roasted almost one hour, didn't want the edges of the florets to get too dark. I didn't have chicken broth or bouillon cubes so I used a chicken flavoring packet from ramen noodles. After the first blend in the blender, I decided to drain the liquid into a larger measuring cup and puree the cauliflower bits in my Nutri-bullet. I had to add half of the liquid so as not to create a paste. When I had a comfortable blend, I transferred it to the stove and added the rest of the liquid. I added just a dash of ground thyme (didn't have fresh) and enjoyed a very thick and creamy roasted flavored soup with no lumps. Delish!
I made a half-version and followed other reviewers' comments to reduce roasting time to 30 mins.
In the last fifteen minutes, I simmered 1/4 yellow onion, 2 tsp garlic, and white wine to coat in a pot. I added the cauliflower and 1/2 of the broth, then cooked for another 10 minutes with a cover, then removed from heat for another 10 minutes, uncovered.
I added the other 1/2 of the broth and the milk, blended with immersion blender, then reheated a couple of minutes to serve.
With those revisions, it matched a fabulous, full-fat version I'd had a few days ago.