1. 1. In a medium bowl, mix together sage, rosemary, oregano, garlic, fennel pollen, salt, pepper, and lemon zest; set aside.
2. 2. Place pork belly skin side up on a clean work surface. Using a very sharp knife, score skin in a diamond-shape fashion. Turn pork belly over so that it is skin-side down. Season flesh side of belly with salt and pepper and half of the herb mixture.
3. 3. Place pork loin on a cutting board. Holding the blade of the knife parallel to board, cut along the length of the pork loin, but not all the way through. Unfold so that it opens like a book. Place butterflied loin on top of pork belly. Season with salt and pepper and spread with remaining herb mixture.
4. 4. Roll pork away from you into a cylindrical shape; tie with kitchen twine at 1-inch intervals to make snug.
5. 5. Season skin with salt and pepper. Transfer to refrigerator, uncovered, so that it air-dries for 3 days.
6. 6. Fit a rimmed baking sheet with a rack and transfer pork to rack. Preheat oven to 500 degrees with a rack set in the lower third of the oven.
7. 7. Transfer pork, skin side up, to oven and cook until skin begins to crackle, 20 to 25 minutes. Reduce temperature to 325 degrees and continue roasting until an instant-read thermometer inserted into center of roast reaches 135 degrees, about 2 1/2 hours, tenting with foil if necessary. Let stand 30 minutes before slicing.

For the Sandwich

1. 1. Make the salsa verde: In a medium bowl, mix together shallot, parsley, chives, vinegar, oregano, and salt; whisk in olive oil until emulsified. Stir in mustard and lemon juice, set aside.
2. 2. Make the sandwich: Cover split sides of rolls with salsa verde. Place 3 porchetta slices and some of the crackling on one half of each of the rolls; top with arugula; and sandwich together with remaining half.