In a small frying pan, lightly toast 2 tsp of rosemary needles along with the fennel seeds over low heat, shaking constantly, until fragrant, about 30 second to 1 minute. Let cool and roughly chop into a rough spice blend. Add the fennel and rosemary to a small bowl along with the sea salt, freshly ground pepper, and crushed red pepper (if using). Mix well. In another small bowl, mix the lemon zest and flat leaf parsley together.
Place the pork belly skin side down on a cutting board and lightly score the meat in a diamond pattern. Sprinkle on half of the salt rub. Top generously with all of the herb rub. If using, place the tenderloin in the centre of the belly.
Tightly roll the belly, skin side out, around the tenderloin and tie together with kitchen twine. Rub the skin generously with the remaining salt rub. Make ahead: tightly wrap the porchetta in plastic wrap, place in a dish, and put in the fridge overnight.
Heat the oven to 275°F. Make sure the surface of the porchetta is dry; pat with paper towels if needed. Lightly rub with neutral oil. Place the roll on a rack in a deep roasting pan, seam side down. Roast on the centre rack of the oven for 2-3 hours, basing with pan drippings every 30 minutes. Use a meat thermometer to ensure the internal temperature of the belly reaches 160°F, which is optimal juicy pork belly temp.
Blast the heat up to 450°F and continue to roast for 20-25 minutes, until the crackling turns golden brown and crispy, checking every 5 minutes. Remove from the oven, let rest for 15-20 minutes, slice and enjoy!