On a baking pan lined with parchment paper, scatter the cauliflower. Sprinkle with salt & pepper then drizzle with extra virgin olive oil. Bake in the preheated oven for 30 minutes.
After 30 minutes, over medium heat, sautè the onions and celery in a saucepan with extra virgin olive oil.
Add the roasted cauliflower then toss for about 5 minutes.
Add the vegetable broth and rosemary and cook for another 15 minutes.
With the immersion blender, blend the soup until smooth and creamy in the saucepan. Season with salt and pepper then cook for 5 more minutes.
Serve in mugs then drizzle with extra virgin olive oil before serving.