Place the pork belly, skin side down, on a work surface and place the pork loin in the center. Roll the belly around the loin so that the short ends of the belly meet. If any of the belly or loin overhangs, trim the excess. Unroll the belly and loin.

Refrigerate the pork, uncovered, for 1 day to allow the skin to air-dry, patting it occasionally with paper towels.

The next day, remove the porchetta from the refrigerator and let stand at room temperature for 2 hours. Meanwhile, in a small fry pan, toast the fennel seeds and red pepper flakes until fragrant, about 1 minute. Tip them into a spice grinder and let cool, then finely grind them. Transfer to a small bowl and stir in the sage, rosemary and garlic.

Preheat the oven to 500°F. 

Place the pork belly, skin side down, on a clean work surface. Using a knife, score the flesh in a checkerboard pattern, 1/3 inch deep, so that the roast will cook evenly. Turn the belly skin side up. Using a paring knife, poke dozens of holes, each 1/8 inch deep, through the skin. Using a meat mallet, pound the skin for 3 minutes to tenderize the meat.

Turn the belly and generously season both the belly and the loin with salt. Rub both pieces with the spice mixture. Arrange the loin down the middle of the belly. Roll the belly around the loin and tie it crosswise with kitchen twine at 1/2-inch intervals. Trim the twine and transfer the pork to a wire rack set inside a rimmed baking sheet.

Roast the pork for 40 minutes, then turn the meat over. Reduce the oven temperature to 300°F and continue roasting, rotating the pan and turning the porchetta occasionally, until an instant-read thermometer inserted into the center of the meat registers 145°F, 1 1/2 to 2 hours more. If the skin is not yet deep brown and crisp, increase the oven temperature to 500°F and roast for 10 minutes more.

Let the porchetta rest for 30 minutes. Using a serrated knife, slice it into 1/2-inch  rounds and serve.