Cut cauliflower into individual florets. Season generously with salt and pepper and toss with olive oil. Spread onto a baking sheet into a single layer. Roast for 25 minutes or until golden brown and softened.
While cauliflower is roasting, in a large pot or dutch oven cook onions in butter on medium-high heat until softened. Add in flour stir to fully incorporate.
Add vegetable stock and stir until thickened. Bring to a simmer and add cauliflower.
Puree soup with an immersion blender (a stand blender is fine, but be careful processing hot soup!) and blend until smooth. If you want a chunky soup, reserve some cauliflower to add in after the puree.
Season soup with thyme and with salt and pepper to taste. Stir in cream, then serve and enjoy!