How to Make It
In a small bowl, combine the porcini mushrooms with the sugar, cover with hot water and let soak until the mushrooms have softened, about 30 minutes. Drain and chop the porcini.
Meanwhile, in a food processor, combine the onions, fennel, celery, carrot and garlic and pulse until very finely chopped.
In a large enameled cast-iron casserole, heat the oil. Add the pancetta and cook over moderate heat, stirring, until browned, about 5 minutes. Using a slotted spoon, transfer the pancetta to a plate.
Season the chicken with salt and pepper and add it to the casserole. Cook over moderately high heat, turning once, until golden brown, about 10 minutes. Transfer the chicken to a platter.
Pour off all but 2 tablespoons of fat from the casserole. Add the chopped vegetables and a generous pinch each of salt and pepper; cook over moderately low heat until the vegetables are softened and just starting to brown, about 15 minutes. Stir in the rosemary, sage, tomato paste and porcini. Add the vinegar and cook over moderate heat until almost evaporated, about 3 minutes. Add the wine and cook, stirring, until reduced by half, about 3 minutes. Add the stock and tomato sauce and bring to a boil.
Return the chicken thighs to the casserole. Cover partially and simmer over low heat until the chicken is very tender, about 1 hour. Transfer the chicken to a platter and let cool slightly, then remove the meat from the bones. Shred the chicken and stir it into the sauce along with the olives and reserved pancetta. Season the ragù with salt and pepper and keep warm.
In a large pot of salted boiling water, cook the pappardelle until al dente. Drain well. In a large bowl, gently toss the pappardelle with the ragù and serve, passing freshly grated cheese at the table.