While the ingredients and preparation here may seem at first glance to be Italian in origin, the cuisines of Mediterranean France and Italy are one and the same. There is no more delicious or authentic vegetarian French dish, in my mind. I simply toss fresh tagliatelle—Provence's favorite pasta—with a sauce made from the water used to reconstitute dried porcini mushrooms, the porcinis themselves, and crème fraîche, studded with crisp-roasted button mushrooms, garlic, and thyme. The sauce is so flavorful that it immediately and visibly stains the tender ribbons of pasta as soon as you toss them together. Such a keeper.