From http://skinnygourmet.blogspot.com/2008/12/healthy-meets-indulgence-cheddar-and.html, but I added fat back in and made it lower carb by replacing the flour/skim milk with heavy cream. This looks amazing and I'm looking forward to trying it soon.
Put the carrot (peeled but not chopped) and the cauliflower (also one big hunk) into a large pot with 2 cups of chicken broth. Bring the broth to a boil then reduce to a low boil. Add the broccoli and steam until the broccoli is al dente.
Drain the broth into a liquid measuring cup. Add more chicken broth to increase the total volume to 3 cups. Remove the cauliflower and carrot (which should be well softened) and puree. Set aside.
Add butter to a skillet, and saute diced onion, pepper flakes and garlic over medium low until softened and slightly carmelized. Add cream. Cook over medium for four minutes.
Remove from heat. Stir in the reserved chicken broth. Return to heat over medium and bring to slow boil. Boil 1 minute then remove from heat. Stir in the vegetable puree. Slowly add the shredded cheese in small amounts, stirring to combine. Add salt and pepper to taste.