Heat a nonstick saute pan over medium-low heat. Add 2 tablespoon fennel seeds and toast until aromatic, about 1 minute. Remove from heat and cool. Grind in food mill until fine.
Finely chop 2 tablespoon parsley and 1 tablespoon rosemary. Combine with ground fennel, red pepper flakes, lemon zest, garlic, salt, pepper, and olive oil.
Prepare the pork. Using the tip of a sharp knife, cut ½ inch shallow slits across the skin of the pork. Repeat on the diagonal, creating lines that crisscross. Flip the pork over and repeat on the flesh side. Spread the herb mixture on the flesh side of the pork. Tightly roll the pork with the skin facing side out. Bind the pork together with butcher's twine. Season the skin with 1 tablespoon 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 2 hours or until internal temperature reads 145 degrees.
Meanwhile, make the salsa verde. Finely chop the parsley and combine with toasted fennel, ground coriander, red pepper flakes, garlic, the juice and zest of 1 lemon, and olive oil. Season with salt and pepper and set aside.
Increase temperature to 450 degrees and cook an additional 15 minutes or until internal temperature reads 160 degrees and 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. Toast ciabatta bread. Stack with sliced porchetta, drizzle salsa verde, and top with cracklings. Serve.