Place the fennel seeds in a small pan or skillet over medium heat and toast shaking the pan a few times, until fragrant and lightly golden, 3 to 4 minutes. Immediately transfer to a mortar and pestle, spice grinder, or mini-prep food processor and pound or grind until seeds are finely ground.
Transfer the ground toasted fennel to a food processor. Add the garlic, rosemary, thyme, sage leaves, and 1½ tablespoons salt and ½ teaspoon pepper. Puree for 30 seconds, scrape down the sides, and pulse 10 times.
Add the lemon zest and pepper flakes, pulse a few more times to combine. Set aside.
If tied, untie pork and use a very sharp knife to score the pork skin, making cut lines that are about 1-inch apart and no more than 1/8-inch deep.
Pat the pork shoulder completely dry with paper towels. Rub the pork shoulder all over on both sides with the herb mixture.
Gently roll up the pork, with skin on the outside, then tie it crosswise with kitchen twine at 3/4 to 1-inch intervals, keeping as much of the herb mixture inside the pork roll as possible. Refrigerate on a large plate or a baking dish, uncovered, overnight.
Preheat oven to 425°F (200°C).
Place tied pork shoulder in a roasting pan, fat-cap side up. Rub the pork all over with the olive oil, pour the broth into the pan, and cook until the skin starts to crisp, 35 to 40 minutes.
Reduce the heat to 300°F (150°C), baste the pork with pan juices, then continue to cook, basting every 30 minutes, until the skin is well browned, the meat is fork tender, and the internal temperature is 150°F to 155°F, 1½ to 2 hours more.
Remove the pork from the oven, turn the oven to broil, then return the pan to the oven and broil the meat 4 to 5 inches from the heat source just until the skin is extra crisp, 1 to 2 minutes.
Remove the meat to a cutting board and let rest for 20 minutes. Carve with a serrated knife, and serve.