Preheat the oven to 400 degrees F.
Toss the cauliflower with 2 tbsp olive oil in a bowl and transfer to a baking sheet. Sprinkle with salt and bake for about 25 minutes.
Heat 2 tbsp olive oil in a large pot and cook the onion and garlic, stirring occasionally, for about 7 minutes.
Add the roasted cauliflower, nutmeg, pepper and vegetable broth and bring to a boil.
Boil for 15 minutes, then cool and transfer to a blender to blend until creamy, then reheat before serving.