In a food processor, combine the onions, fennel, celery, carrot and garlic and pulse until very finely chopped.

In a large enameled dutch oven, 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 dutch oven. Cook over moderately high heat, turning once, until golden brown, about 10 minutes. Transfer the chicken to a platter. The chicken will not be cooked through yet.

Pour off all but 2 tablespoons of fat from the pot. 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, tomato paste, and hot chili peppers. Add the balsamic vinegar and cook over moderate heat until almost evaporated, about 2 minutes. Add the wine and cook, stirring, until reduced by half, about 3 minutes. Add the stock and Colavita tomatoes 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, 30 minutes. Transfer the chicken to a platter and let cool slightly. Shred the chicken and stir it into the sauce along with the reserved pancetta. Season the sauce with salt and pepper if needed 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.

Garnish with freshly chopped parsley and grated Parmigiano-Reggiano cheese.