Stuffed Zucchini with Mortadella and Parmesan

Name: Stuffed Zucchini with Mortadella and Parmesan
Ingredients: 3 tablespoons olive oil, divided, plus additional for baking dish; 1 1/2 cups chopped white onion; 2 garlic cloves, chopped; 4 ounces mortadella, minced (3/4 cup); 1 cup fresh breadcrumbs (from crustless Italian or French bread ground in processor); 1 cup finely grated Parmesan cheese, divided; 1/2 cup chopped fresh Italian parsley; 1 large egg, beaten to blend; 8 medium round zucchini, tops cut off, or 4 large zucchini (7 to 8 ounces each), trimmed, cut lengthwise in half