In a saucepan, heat the remaining 2 tablespoons of oil. Add the onion and garlic and cook over moderate heat until translucent, 6 minutes. Add the farro, porcini and their soaking liquid, the remaining 5 cups of stock and the bay leaf. Bring to a simmer, then cover and cook over moderately low heat until the farro is tender, about 20 minutes. Drain well; discard the bay leaf.