This soup from southern Tuscany is quintessentially autumn, starring two of the season’s most representative ingredients: chestnuts and mushrooms. I picked it from the very traditional menu of a trattoria in Monte Amiata, an ancient and now dormant volcano that dominates the surrounding landscapes: the Val d’Orcia, Bolsena lake, the Chianti hills, and the Maremma plains. It was cold outside, and we had just had a walk around Piancastagnaio, a town famous for its tile-roofed houses, clinging to the side of a cliff, and its ancient chestnut trees, after which it is named. The trees surround the town and gift the villagers one of the most important ingredients of their local cuisine—chestnuts—celebrated every year in the famous Crastatone festival. That soup, combining chestnuts with potatoes and porcini mushrooms, another prized local product, was screaming my name. Since that day, I’ve made this soup often, with fresh porcini when I am …