Score the skin of the pork belly in a diamond pattern; avoid cutting all the way through to the meat.
Combine garlic, olive oil, lemon zest, fennel seeds, coriander, sage, rosemary leaves, oregano, and cayenne pepper in a blender; pulse into a paste.
Rub pork belly and pork loin with salt. Flip pork belly so the skin is facing down. Place the loin in the center and wrap the belly around it. Cut off any overlapping ends of the belly and any overhanging portions of the loin; you want a nice, neat package.
Unwrap the belly from the loin. Rub all sides of the loin with the paste; don't skimp-use all of it. Re-roll the belly around the loin. Pat the skin dry with a paper towel and use kitchen twine to tie it off in 2-inch segments. (This will make it hold its appealing cylinder shape as it cooks.)
Place roast on a wire rack on top of a rimmed pan or pot. Place in the refrigerator, uncovered, until the skin is air-dried, about 48 hours.
When ready to cook, remove roast from the refrigerator. Transfer to a roasting pan and let sit for 1 hour.
Preheat the oven to 500 degrees F (260 degrees C).
Place the roast in the lower half of the preheated oven and cook for 30 minutes. Reduce heat to 300 degrees F (150 degrees C) and cook until meat is tender and an instant-read thermometer inserted in the center reads 145 degrees F (63 degrees C), about 2 1/2 more hours.
Remove from the oven and tent with foil; let sit for 30 minutes. Remove twine and cut into 1-inch slices. Serve immediately.