Cut ends off zucchini, halve lengthwise, and with a melon baller or small spoon, scoop out the flesh into a bowl. Place the zucchini “boats” on a cookie sheet or jelly roll pan. Chop the flesh and return it to the bowl.
Heat about ¼-cup olive oil in a skillet over medium heat. Add onion and garlic and cook 5-8 minutes. Add zucchini flesh and cook 10-15 minutes or until the onion starts to brown and the liquid is evaporated. Salt and pepper to taste.
Combine cooked onion, garlic, and zucchini mixture in a bowl. Add bread crumbs, Parmesan, egg, and rosemary. Mix thoroughly.
Spoon the zucchini mixture into the boats. Bake them cook for approximately 25 minutes, until the zucchini boat is tender and the tops are browned.
Serve warm. Optional: Add more Parmesan on top of boats and put in a broiler to melt the cheese.