How to Make It
Melt the butter in a heavy saucepan. Add the onions and cook over moderately low heat, stirring occasionally, until very soft but not browned, about 15 minutes.
Add the cauliflower and chicken stock and bring to a boil. Cover and simmer over moderately low heat, stirring once, until the cauliflower is very soft, about 30 minutes. Let cool slightly.
Working in batches, puree the soup in a blender. Return the soup to the saucepan, stir in the cream and season with salt and pepper. If necessary, add a few tablespoons of water to thin the soup.
Preheat the oven to 325°. Heat a large ovenproof skillet. Brush the mushrooms with the oil, season with salt and pepper and add them to the skillet. Sear over moderately high heat, turning once, just until tender, 6 to 8 minutes. Transfer the skillet to the oven and bake for 3 to 4 minutes, or until the mushrooms are very tender. Thinly slice the mushrooms.
Rewarm the soup over moderate heat. Ladle into shallow bowls, garnish with the sliced mushrooms and serve.