Preheat the oven to 400 degrees F.Place the cauliflower florets onto a lined baking sheet and drizzle with the olive oil and a pinch of salt. Roast for 20-25 minutes until nicely browned, turning with a spatula after 15 minutes. Set aside until ready to use.
Heat the butter in a stock pot over medium-high heat and cook the onion until soft and translucent, 5-7 minutes. Add the remaining vegetables and cook for another 5-7 minutes until softened. Add the roasted cauliflower, stock, thyme, bay leaf, salt and pepper and bring to a boil. Reduce the heat to medium-low, cover and simmer for 25 minutes. Discard the bay leaf.Use an immersion blender to puree the soup to consistency you prefer (I like to puree just part of it and leave the rest chunky). Alternatively transfer a portion of the soup to a regular blender, puree it, and add it back to the pot. Add the cream and parsley and heat through. Add salt and pepper to taste.Note: If you prefer a thicker consistency you can dissolve some cornstarch in a little water (e.g., 1 tablespoon cornstarch in 3 tablespoons water) and pour it into the soup at the end. Let the soup simmer for a minute or two until slightly thickened.