Cook shells according to package directions. Drain then carefully rinse with cold water; drain again.
In a large skillet, cook ground beef, onion, and garlic until meat is brown; drain.
In a medium bowl combine egg, bread crumbs, fennel seeds, salt, and black pepper. Add the meat mixture and 1/2 cup of the spaghetti sauce; mix well. Spoon about 2 tablespoons of the meat mixture into each macaroni shell. Arrange the filled shells in a 2-quart oblong baking dish. Pour the remaining sauce over the filled shells. Cover the dish with foil.
Bake 375° F for 35 to 40 minutes or until heated through. Sprinkle with mozzarella cheese and place back in the oven, uncovered, until cheese is melted.