Rinse duck with cold water and pat dry of excess moisture with paper towel. Carefully pull the skin from the flesh, without actually taking it off the duck, to create a pocket for the rub. Set duck aside.
For the stuffing:
Heat olive oil in a large skillet over medium heat. Add diced pancetta and cook until fat renders. Add onion, fennel, chestnuts, fennel seeds, rosemary, sage, chili flakes, and black pepper, tossing to combine. Cook until onions have browned. Remove from heat and mix together with sausage and Italian parsley. Stuff cavity of duck with sausage mixture.
For the rub:
Combine chopped fennel, garlic, fennel seeds, rosemary, sage, dried chili flakes, and pepper in a small bowl. Distribute the rub underneath the skin of the duck, all over the flesh. Once again, pat the duck skin dry of excess moisture. Truss the duck with butcher’s twine. Rub the skin with 1/2 cup (120 milliliters) olive oil and sprinkle generously with salt.
Sear on all sides in a cast-iron pan, ending with breast-side down. Transfer to oven and cook for 90 minutes, basting every 30 minutes, then cover with tin foil and cook for one more hour, basting every 30 minutes.
Transfer duck to cutting board and cover in tinfoil. Let duck rest for 20-30 minutes before carving.