1 Halve the zucchini lengthwise.
2 In a large skillet, bring the broth, garlic, basil and pepper to a boil over medium-high heat. Add the zucchini halves, reduce the heat to low, cover and simmer until barely tender, about 5 minutes.
3 Remove the zucchini and set aside to cool slightly. Measure out and remove 1/4 cup of the broth, leaving the remainder in the skillet.
4 Increase the heat under the skillet to medium. Crumble in the ground beef and cook, stirring occasionally to break it up, until the meat is no longer pink, about 5 minutes.
5 Preheat the broiler. Line a baking sheet with foil.
6 Meanwhile, carefully scoop out the zucchini flesh, leaving a 1/4 ineh-thick shell. Coarsely chop the flesh.
7 In a small bowl, combine the reserved 1/4 cup of broth with the cornstarch and stir to combine. Bring the beef mixture to a boil over medium-high heat. Add the chopped zucchini and the cornstarch mixture. Cook, stirring, for about 1 minute, or until the liquid has thickened. Remove the skillet from the heat.
8 Coarsely chop the tomatoes and stir them into the beef mixture.
9 Place the zucchini halves on the prepared baking sheet. Dividing evenly, mound the beef mixture into the hollowed-out zucchini and sprinkle with the mozzarella. Broil 4 inches from the heat until the cheese is melted and bubbling, 2 to 3 minutes.