Adjust oven rack to lower-middle position and preheat oven to 300 degrees. Combine salt, rosemary, thyme, oregano, fennel seeds, and pepper flakes in bowl. Tie pork with kitchen twine at 1-inch intervals. Transfer pork to large Dutch oven and season all over with salt mixture. Pour broth around pork and add garlic to pot. Cover, transfer to oven, and cook until meat registers 190 degrees, 2½ to 3 hours.

Transfer pork to large plate. Transfer braising liquid to 4-cup liquid measuring cup; add extra broth if necessary to equal 3 cups. Let pork and liquid cool completely, about 1 hour. Cover and refrigerate both for at least 1 hour or up to 2 days.

Heat oil and garlic in Dutch oven over medium heat until garlic is golden brown, 3 to 5 minutes. Add broccoli rabe, salt, and pepper flakes and cook, stirring occasionally, until tender, 4 to 6 minutes. Transfer to bowl.

About 20 minutes before serving, adjust oven rack to middle position and heat oven to 450 degrees. Remove twine and cut cooled pork in half lengthwise. Position roasts cut side down and slice each crosswise as thin as possible.

Spoon solidified fat off cooled jus and discard. Transfer jus to Dutch oven and bring to boil over high heat. Reduce heat to low, add pork, cover, and cook until pork is heated through, about 3 minutes, tossing occasionally. Cover and keep warm.

Arrange rolls on 2 rimmed baking sheets (4 rolls per sheet). Divide provolone evenly among rolls. Bake, 1 sheet at a time, until cheese is melted and rolls are warmed, about 3 minutes. Using tongs, divide pork and broccoli rabe evenly among rolls (about 1 cup pork and ⅓ cup broccoli rabe per roll). Serve.