Lay the pork belly flat on a clean surface, skin side down. Score the meat in a crosshatch pattern to allow the seasonings to penetrate.
In a mortar and pestle or spice grinder, crush the fennel seeds and peppercorns until they are a coarse powder. Mix this with the kosher salt, minced garlic, rosemary, thyme, sage, and lemon zest.
Rub the seasoning mixture evenly over the scored side of the pork belly. Roll the pork belly tightly into a cylinder, skin side out, and tie it securely with kitchen twine at 1-inch intervals.
Cover the rolled pork belly with plastic wrap and refrigerate for at least 4 hours, preferably overnight, to allow the flavours to meld.
Preheat your oven to 325°F (165°C). Place the rolled pork belly on a roasting rack set in a roasting pan. Drizzle with olive oil and rub it into the skin. Roast for about 2.5 to 3 hours, or until the internal temperature reaches 145°F (63°C).
Increase the oven temperature to 450°F (230°C) and roast for an additional 20-30 minutes, or until the skin is golden brown and crispy.
Remove the porchetta from the oven and let it rest for 20 minutes before slicing. Serve with your favourite side dishes and enjoy!