Score the skin of the pork belly with an extremely sharp knife or a razor blade (or ask your butcher to do it). Poke the flesh side all over with the tip of a paring knife.
Put the fennel seeds and peppercorns in a small dry skillet; toast over medium heat until very fragrant and a few shades darker, about 3 minutes. Transfer to a plate to cool, then grind in a spice grinder. Mix the ground spices with the red pepper flakes, sage, rosemary, thyme, lemon zest, orange zest, garlic and 2 tablespoons salt. Rub the mixture into the flesh side of the pork belly.
Roll the belly up as tightly as possible and secure it with kitchen twine at 1-inch intervals. Rub the outside with the baking powder and 1 tablespoon salt. Refrigerate, uncovered, for 1 to 2 days.
Preheat the oven to 325 degrees F. Set the rolled pork belly in a large roasting pan and roast until the internal temperature reaches 180 degrees F on an instant-read thermometer, about 2 hours. Increase the oven temperature to 500 degrees F and roast until the skin is golden brown and crisp all over, about 20 minutes more. Transfer to a cutting board and let rest 10 minutes before slicing.