Sauté in a pan, over medium heat - onions, fennel bulb, celery & garlic in olive oil until softened. Set aside to cool.
Score the skin side of the pork belly crisscrossing every 1-2 inches. Cover with plastic wrap & flip over. Season the meat side of the pork belly with the ground fennel, oregano & chili flakes. Season with salt & freshly cracked black pepper.
Over the seasonings layer the pancetta, top with the cooled onion & fennel bulb mixture. Lay the pork loin in the centre of the pork belly lengthwise. Tie the porchetta up at one inch intervals to create a uniform roll.
In a food processer pulse the zest & rosemary, garlic & olive oil to create a paste. Slather the paste over the porchetta. Work the herb rub into the scored skin. Season the porchetta generously with salt & freshly cracked black pepper. Cover with plastic wrap & marinate overnight.
Smoke the porchetta cut side down for 4 hours at 225°F then increase heat to 375°F for the last ½ - 1 hr or until the skin is crispy. Slice and serve.