Preheat the oven to 375°F. Cut each zucchini in half lengthwise and scoop out the pulpy center with a teaspoon, leaving a 1/2-inch shell. Roughly chop the pulp.
Place the zucchini shells in a shallow baking dish, cover with foil and bake for 20 minutes, or until tender when pierced.
Meanwhile, cook the rice in a saucepan of lightly salted boiling water for 12 to 15 minutes until just tender. Drain, allow to stand for 5 minutes. Add the tomatoes, egg, mint, if using, and salt and pepper to taste. Set aside.
Sauté the lamb over medium heat for 5 minutes, or until browned. Add the onion and garlic and cook for 5 minutes more until the onion has softened. Stir in the tomato sauce, reserved zucchini pulp, and rosemary. Bring to a boil, then reduce the heat and simmer gently for 10 to 15 minutes. Stir in the rice mixture and mix well.
Spoon the stuffing mixture into the zucchini shells and sprinkle with Parmesan. Bake for 15 to 20 minutes until golden and crispy on top. Serve immediately.