1. Using a mortar and pestle, crush the fennel seed. Add the garlic, salt, sugar, pepper, and sage. Crush until mixture becomes a rough paste.
2. Open pork roast and lay flat side down; cut the meat if necessary to make it lie flat. Rub surface of the roast with about half the garlic paste. Lay liver strips lengthwise over meat.
3. Roll the roast tightly lengthwise with seasoned surface inside. Tie with heavy string at 2-inch intervals. Rub remaining garlic paste on outside of roast. Place roast on a rack in a shallow baking pan.
4. Cook, uncovered, at 375°F until meat thermometer inserted in thickest part of the roast registers 1 70°F (about 1 1/2 hours). Transfer roast to a serving platter and keep warm.
5. Remove rack from roasting pan and place pan over direct heat. Stir together the cornstarch and broth until blended. Stir into drippings in roasting pan. Cook over medium heat, stirring constantly, until sauce boils and thickens. Pour sauce into a serving bowl.