For the herb rub: Add the garlic, fennel seeds and red pepper flakes to a food processor and pulse until finely chopped. Add the oregano, rosemary and thyme and pulse into a rustic paste. Season with salt. Reserve 2 tablespoons of the rub for the mayonnaise spread.
For the pork: Lay the pork belly on a cutting board, skin-side-up, and score the skin at 1-inch intervals, 1/2-inch deep with a sharp knife. Rotate the pork 90 degrees and score the skin again, to create a diamond pattern. Sprinkle generously with salt; be sure to get into all the crevices. Flip the pork over, sprinkle generously with salt, then rub the meat side with the herb rub. Roll the pork belly up as tightly as possible and tie it crosswise with butcher's twine at 2-inch intervals.
Place the pork in a roasting pan fitted with a rack. Pour 1/2 cup water into the bottom of the pan. Roast the pork to an internal temperature of 150 degrees F, 2 1/2 to 3 hours. Check periodically to ensure there is still some liquid in the bottom of the pan and add a little water to prevent scorching. Baste the pork belly with pan drippings a couple of times during cooking.
Once the pork reaches 150 degrees F, raise the oven to 475 degrees F and continue roasting until the skin is super crispy, 30 to 40 minutes. For even browning, flip the pork halfway through the second roasting period.
Transfer the porchetta to a cutting board and let rest for 20 to 30 minutes.
Remove the rack from the roasting pan and pour off all but 3 tablespoons of the pan drippings. Set the roasting pan on the stovetop over medium heat. Add the onions and saute until lightly caramelized, about 15 minutes. Season with salt and pepper.
For the mayonnaise spread: Mix together the mayonnaise, capers, parsley, lemon zest and juice and the reserved herb rub. Season with salt and pepper.
Cut the porchetta into 1/4-inch-thick slices.
To assemble the sandwich: Slather both halves of a ciabatta roll with mayonnaise spread. Layer the bottom half with slices of porchetta and top with caramelized onions and arugula.