Combine ground fennel, parsley, rosemary, red pepper flakes, lemon zest, garlic, 2 teaspoons salt, pepper, and olive oil in a medium bowl.
Prepare the pork. Using a sharp knife to butterfly the pork, making sure not to cut all the way through. Cut ½ inch shallow slits across the flesh of the pork. Repeat on the diagonal, creating lines that crisscross. Spread the herb mixture evenly on the flesh and tightly roll the pork with the skin facing side out. Bind the pork together with butcher's twine. Season the skin with 3 teaspoons of salt evenly on all sides and refrigerate uncovered overnight, patting dry excess moisture with paper towels.
Preheat oven to 325 degrees F.
Remove the pork from the refrigerator and let sit at room temperature for 1 ½ hours. Place on roasting rack and roast in the oven for 1 hour or until internal temperature reads 145 degrees.
Meanwhile, make the salsa verde. Combine parsley, fennel, coriander, red pepper flakes, garlic, the juice and zest of 1 lemon, and olive oil in a medium bowl. Season with salt and pepper and set aside.
Increase the oven temperature to 450 degrees. Cook the pork for an additional 15 minutes or until the internal temperature reads 160 degrees and the skin is crackling. You may need to rotate the pork while roasting to allow even cooking.
Remove porchetta from the oven. Let rest for 25 minutes. Carve into thin slices.
Assemble the sandwiches. Stack ciabatta buns with sliced porchetta, drizzle salsa verde, and top with cracklings and the remaining buns. Serve immediately.
Notes
You can make the salsa verde the day before, but the color may dull over time.For a more traditional version, use pork belly instead of pork shoulder.