Preheat the oven to 190 degrees C. Cut each zucchini in half lengthways and scoop out the pulpy centre with a teaspoon, leaving an outside shell 1cm thick. Reserve the pulp, and chop lightly.
Place the zucchini in a shallow baking dish or roasting tin, cover with foil and bake for 20 minutes or until tender – they should pierce easily with a fork.
Meanwhile, cook the rice in a pan of lightly salted boiling water for 12–15 minutes until just tender. Drain, allow to stand for 5 minutes, then add the chopped tomatoes, egg, mint and pepper to season. Set aside.
Fry the beef over a medium heat for 5 minutes or until browned. Add the onion and garlic, cook for a further 5 minutes until the onion has softened. Stir in the passata, reserved zucchini pulp and rosemary. Bring to the boil, then reduce the heat and simmer gently for 10–15 minutes. Stir in the rice mixture and mix together.
Spoon the stuffing mixture into the zucchini ‘boats’ and sprinkle with Parmesan. Bake for 15–20 minutes until golden and crispy on top. Serve immediately.