How to Make It
In a large skillet, heat 2 tablespoons of the oil until shimmering. Add the yellow onion and sliced fennel and cook over moderate heat, stirring occasionally, until softened, 10 minutes. Add the sausage, fennel seeds, rosemary, garlic and the 1 tablespoon of pepper and cook, breaking up the meat, until the sausage is browned, about 5 minutes. Season with salt. Transfer the mixture to a bowl and let cool. Stir in the eggs and fennel fronds.
Preheat the oven to 350°. Place the red onions in a large roasting pan. Arrange the pork on a work surface and season with salt and pepper. Spread the sausage mixture over the pork, leaving a 1-inch border all around. Starting at a long side, roll up the pork and tie with kitchen twine at 1-inch intervals. Rub with the remaining 2 tablespoons of oil and place on top of the red onions. Roast the pork for about 2 1/2 hours, basting occasionally with any pan juices, until an instant-read thermometer inserted in the center of the meat registers 160°. Transfer to a cutting board and let rest for 15 minutes.
To serve, discard the strings. Cut the pork into 1-inch-thick slices and serve with the red onions.