You can make this make-ahead meal in two strategies: Prepare and tie the pork, and trim the rabe and store. Then roast the pork and parboil the broccoli rabe on the day you want to serve. Or roast the pork and parboil the broccoli rabe a few days before and reheat both the sliced pork and rabe in simmering chicken broth.
For the pork and broccoli rabe: Butterfly the pork, cut the meat across, but not all the way through, and open the shoulder up like a book. If the meat is a little uneven, then pound it out to even. Sprinkle the pork evenly across the surface with the rosemary, fennel seeds, red pepper flakes, garlic and some salt and pepper. Grate the onion over the meat with the large side of a box grater or with a wide tooth fine grater and spread it around the meat. Roll and tie the pork with kitchen string to secure. If using, rub the skin with the baking soda for extra crispy skin. Drizzle or spray with a little oil and sprinkle with salt and pepper.
Roast the pork on the center rack, covered with foil, to an internal temperature of 145 degrees F, about 2 1/2 hours. Remove the foil and broil to crisp the skin, 15 to 20 minutes. Let the pork rest before slicing.
Bring a few inches of water to a boil in a deep skillet. Salt the water, then add the rabe and boil to cook out the bitterness, 5 to 7 minutes. Drain.
For the gremolta: Place the lemon zest on a cutting board and pile the parsley, garlic and chiles on top. Finely chop them all together to combine them. Cut the zested lemon into 10 wedges.
Toast the split ciabatta and drizzle with EVOO. Arrange 3 to 4 thin slices of pork in sandwich and douse with a squirt of lemon juice, top with gremolata and rabe and set tops in place.