Preheat your oven to 325°F (165°C).
Lay the pork belly flat on a cutting board. Score the skin in a crisscross pattern using a sharp knife.
In a bowl, combine the minced garlic, rosemary, sage, fennel seeds, lemon zest, salt, and black pepper. Mix well to create a herb and spice mixture.
Rub the herb and spice mixture all over the pork belly, ensuring it's coated evenly.
Place the pork loin on top of the seasoned pork belly.
Roll the pork belly and loin tightly together, forming a cylinder shape. Use butcher's twine to tie the porchetta securely.
Place the porchetta on a rack in a roasting pan. Drizzle olive oil over the top and season with additional salt and pepper.
Roast the porchetta in the preheated oven for about 3-4 hours, or until the internal temperature reaches 145°F (63°C).
Once cooked, remove the porchetta from the oven and let it rest for about 20 minutes before slicing.
Slice the porchetta into thick, juicy pieces and serve as a main course or in sandwiches.