Fresh white truffles are one of nature's gifts; their earthy taste is a good match for the nutty cauliflower. The puree also tastes delicious drizzled with white-truffle oil.
Place a steamer basket in a medium saucepan filled with 1 inch of water; bring water to a boil, and add the cauliflower. Steam until very tender when pierced with the tip of a knife, about 15 minutes. Repeat with the celery root in the steamer; steam until very tender, about 15 minutes. Set aside.
Melt the butter and the mascarpone in a small saucepan over low heat. Transfer the cauliflower and the celery root to the bowl of a food processor. Drizzle in the warm butter mixture, and process just until the mixture is smooth. Season with salt and pepper to taste. The mixture may be made up to this point and held for 30 minutes in the top of a double boiler, covered, over simmering water. Transfer the puree to a warm serving bowl. Shave the truffle, if using, over the puree. Serve hot.