Preheat oven to 375 degrees. Butter 80-inch square baking pan. Steam zucchini until tender, about ten minutes. Transfer to large bowl and mash coarsely with fork. Spoon zucchini into sieve and drain well, pressing to release excess water. Place drained zucchini in bowl.
Melt butter in heavy large skillet over medium-high heat. Add onion and garlic and saute until light brown, about ten minutes. Add onion mixture to zucchini in bowl. Add eggs, ½ cup Parmesan, breadcrumbs and basil; stir to blend well. Season with salt and pepper. Transfer mixture to prepared baking dish. Bake until firm in center and brown on top, about 45 minutes. Cut into squares and serve, passing additional Parmesan separately.
Can be made the night before and just reheated in water bath. Freezes well. Garnish with fresh basil. Enjoy!
Serves 9 side dishes