How to Make It
Preheat the oven to 375°. On a work surface, cut one-fourth of the cauliflower into 1/2-inch florets. Coarsely chop the remaining cauliflower.
In a large saucepan, heat 2 tablespoons of the olive oil. Add the leek, garlic and a generous pinch of salt and cook over moderate heat, stirring occasionally, until softened, 5 minutes. Stir in the potato and the chopped cauliflower, then add the wine and cook over high heat until reduced by half, 4 minutes. Add the stock and the thyme bundle and bring to a boil. Reduce the heat to moderately low and simmer, stirring occasionally, until the vegetables are very tender, about 30 minutes. Discard the thyme bundle.
Meanwhile, at either end of a large rimmed baking sheet, separately toss the cauliflower florets and brussels sprout leaves each with 1 tablespoon of olive oil and season with salt. Roast for 15 to 18 minutes, stirring each halfway through, until lightly browned and tender. In a small bowl, mix the goat cheese with the chives and chopped thyme.
In a blender, puree the soup in 2 batches until very smooth. Return the soup to the saucepan and stir in the cream. Rewarm over moderate heat, adding water if the soup seems too thick; season with salt. Ladle into bowls and top with the roasted brussels sprout leaves and cauliflower florets. Sprinkle the herbed goat cheese on top and serve hot.