Preheat the oven to 325°F.
Lay the pork belly skin side down on a cutting board. Using a sharp knife, score the flesh of the pork belly in a cross-hatch pattern, making sure not to cut all the way through the meat.
In a small bowl, mix together the fennel seeds, rosemary leaves, minced garlic, coarse salt, black pepper, red pepper flakes, and lemon zest. Add the olive oil and stir until the mixture forms a paste.
Spread the paste evenly over the flesh side of the pork belly, making sure to get it into all the scored areas.
Roll up the pork belly tightly, starting from the short end. Tie it securely with kitchen twine at intervals of about 2 inches.
Place the porchetta on a rack set inside a roasting pan, with the seam side down. Roast for about 3-4 hours, or until a meat thermometer inserted into the thickest part of the pork reads 145°F.
Increase the oven temperature to 500°F and continue to roast for another 10-15 minutes, or until the skin is crispy and golden brown.
Remove the porchetta from the oven and let it rest for at least 15 minutes before slicing and serving.