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.