Cut each zucchini in half lengthwise, then scoop out the flesh to create a ¼-inch shell. Brush the cut sides with two teaspoons of olive oil and set aside. Finely chop the scooped-out zucchini flesh.
In a large skillet, saute the zucchini flesh and onion in the remaining oil. Add the minced garlic and cook for one minute longer. Add the bread crumbs and cook, stirring frequently, until they're golden brown, about two minutes.
Remove the skillet from the heat. Stir in the shredded mozzarella, chopped mint and salt. Spoon the filling evenly into the zucchini shells, then sprinkle each with grated Parmesan cheese.
Grill the stuffed zucchini, covered, over medium heat until the zucchini shells are tender, about 8 to 10 minutes.
