Start making the porchetta 24-hours in advance so the meat can marinate in the herbs overnight and the skin can cure. It’s definitely a good weekend project since the meat also takes a fair amount of time to roast but the house will smell incredible!
Clear a large work area. On a large cutting board, place the pork belly skin side down and arrange the loin in the center. Roll the belly around the loin; if any of the belly overhangs trim the meat. Unroll and set the meat aside.
In a small skillet over medium-low heat, toast the fennel seeds and crushed red pepper until fragrant, about a minute. Remove the spices from the pan into a small mixing bowl, and then add the rosemary, thyme, sage and black pepper. Use a mortar and pestle or spice grinder to grind the spice mixture.
Place the pork belly skin side up on a large cutting board. Use a sharp knife to score the skin and fat in a crosshatch pattern, being careful not to cut down to the meat. Flip the pork belly, so that it is now skin side down. Use a paring knife to poke holes all over the meat.
Season the pork belly on both sides with the kosher salt. Place the pork belly on the cutting board skin side down. Rub the garlic and orange zest all over followed by the spice mixture. Place the loin at the end of the pork belly.
Tightly roll the belly around the loin then tie with kitchen twine in ½” intervals.
Place the tied Porchetta on a platter and refrigerate, uncovered, overnight.
Day 2:
Let the meat sit at room temperature for 1 hour to ensure the meat will cook evenly.
Preheat the oven to 450ºF (230ºC). Transfer the roast to a wire rack set in a rimmed baking sheet. Roast for 35 minutes. Reduce the temperature to 325ºF and roast for an additional 3 hours (a thermometer inserted into the thickest part of the meat reads 180ºF/ 82ºC).
Increase the heat to 500ºF (260ºC) and continue to roast until skin is crispy and golden brown, about 10 minutes.
Remove the roast from the oven and let it rest for at least 20 minutes before slicing. Remove the string and discard.