Add the sausage, fennel seeds, rosemary, garlic and pepper. Break up the sausage into small pieces and cook until browned. Season with salt. Transfer mixture to a bowl and let cool. Stir in the eggs and fennel fronds.
Preheat oven to 350 degrees.
Lay pork on work surface. Season with salt & pepper. Spread cooled sausage & fennel mixture over pork. Starting at a long side, roll up the pork. Tie with butcher's twine at 1-inch spacing. Rub roast with remaining olive oil.
Place red onions into a baking dish. Place pork roast on top. Bake for 2 hours or until instant read thermometer registers 160 degrees in the center. Baste occasionally with any pan drippings. Allow to rest 15 minutes before slicing. Remove strings and cut into 1-inch thick slices to serve.