Prepare the pork: To butterfly, slice down the middle of the pork with a sharp knife, about two-thirds of the way through. Open the meat like a book and pound with a mallet or heavy skillet into a 1-inch-thick rectangle, about
8 by 14 inches. Preheat the oven to 350 degrees. Season the pork with about 1 1/2 teaspoons salt. In a small bowl, mix the lemon zest, garlic, 2 teaspoons fennel seeds, the sage, rosemary and 1 1/2 teaspoons pepper. Rub the mixture all over the pork, then roll it up tightly and tie with twine. Drizzle the roll with olive oil and rub all over; sprinkle with pepper and more fennel seeds. (The roll can be prepared up to 3 days in advance; refrigerate until roasting.)
Place the pork roll on a rack in a shallow roasting pan. Roast until a thermometer inserted into the middle registers 180 degrees and the pork is crusty on the outside, about 2 hours. Let rest 15 minutes before slicing.
While the pork roasts, make the pickled onions: Bring the vinegar, sugar, 1/4 teaspoon salt, the bay leaf, fennel seeds and red pepper flakes to a boil in a small saucepan over medium-high heat. Add the onion slices and simmer 1 minute, then remove from the heat and let the onion cool in the liquid. (Leftover pickled onions will keep in the liquid for weeks.)
Prepare the polenta: Place a large nonstick skillet over medium heat, add the olive oil and fry the polenta until golden and crisp, about 4 minutes per side. Drain on paper towels. To assemble, arrange the polenta on a platter or individual plates. Thinly slice the pork roll, then cut into bite-size pieces and pile on the polenta. Top with pickled onions and serve warm or at room temperature.