Place the cauliflower florets on a baking sheet and toss in olive oil and seasonings. Wrap a head of garlic in foil and add it to the baking sheet. Roast for 20-25 minutes.
In a large pot or Dutch oven, cook the onion and thyme for 5-8 minutes.
Add the broth, roasted cauliflower and roasted garlic to the pot. Bring to a boil then remove it from heat.
Transfer to a blender and blend the soup in two separate batches. Pour it back into the pot and add the cream, spices and cheddar cheese.
Serve the soup immediately, garnished with roasted cauliflower florets and red pepper flakes.