How to Make It
In a small, heatproof bowl, soak the porcini in the hot water until softened, about 15 minutes. Lift the mushrooms out of the soaking liquid and coarsely chop them. Reserve the soaking liquid.
In a large saucepan, heat 3 tablespoons of the olive oil. Add the pancetta and cook over moderate heat, stirring occasionally, until golden brown, about 3 minutes. Add the onion, garlic and thyme and cook, stirring a few times, until the onion is softened, about 5 minutes. Add the chopped porcini along with the parsley and celery leaves and cook, stirring, for 2 minutes. Add the farro and stirring to coat the grains, about 1 minute.
Slowly pour the reserved mushroom soaking liquid into the saucepan, stopping before you reach the grit at the bottom. Add the water and salt and bring to a boil. Cover and simmer over low heat for about 25 minutes, until the liquid is absorbed and the farro is tender but still slightly chewy. Remove the thyme sprigs.
Add the remaining 2 tablespoons of olive oil to the farro along with the beef braising liquid, if using. Stir to combine well and serve.