Prepare the pork. Pat the pork dry and use a sharp knife to score both sides in a diagonal pattern. Line a baking sheet with parchment paper and evenly space 8 pieces of butcher’s string (vertically) across the baking sheet. Lay the pork belly, skin-side-down, over the strings. Set aside.
Saute the veggies. Heat 2 tablespoons of olive oil over medium-high heat in a large, heavy-bottomed skillet. Add the fennel and the onion and saute until the veggies are lightly caramelized and softened.
Make the seasoning mix. Whisk 2 tablespoons of salt together with the pepper, rosemary, parsley, oregano, fennel pollen, lemon zest, orange zest, and garlic. Mix in the sauteed veggies.
Season the pork. Spread the seasoning mix evenly over the pork belly.
Roll. Roll the pork belly around the seasoning. As you roll the pork belly up, sprinkle the skin side with the remaining salt. Use the butcher string to tie the pork tightly around the filling.
Chill. Place the pork belly in the fridge, uncovered. Let it sit for at least 24 hours (ideally 3 days). The skin will dry out during this time so you can get a better crisp on the skin when you roast the porchetta.
Prep. Preheat the oven to 300°F and set a roasting rack over a rimmed baking sheet. Place the pork on the roasting rack and let it sit for 20 minutes.
Roast. Rub the pork with the remaining olive oil and roast for 3 hours or until the internal temperature reads 160°F at the thickest part. Baste the meat with the pan drippings every 30 minutes or so if you can.
Make it crispy. Crank the oven up to 450°F and continue roasting the pork for 25 minutes or until the skin is nice and crispy.
Rest. Remove the pork from the oven and let it rest for 20 minutes before slicing and serving.