Preheat oven to 350°. In a large skillet, heat 2 tablespoons butter over medium-high heat. Add zucchini and onion; cook and stir 10-12 minutes or until crisp-tender. Transfer to a bowl; cool slightly.
In a microwave, melt remaining butter. Drizzle over bread crumbs and toss to coat.
Add the zucchini, onion, garlic salt and egg; mix well.
Stir cheese, eggs, heavy cream, salt and pepper into zucchini mixture; transfer to a greased 8-in. square baking dish. Sprinkle with bread crumb mixture.
Bake, uncovered, 25-30 minutes or until heated through. Let stand 10 minutes before serving.