Heat oven to 350°F. Cut each squash in half lengthwise; cut into 1/2-inch-thick slices. Place squash slices, onions and bell pepper in 11x7-inch (2-quart) glass baking dish or 2-quart casserole. Add 1 tablespoon water. Cover with microwavable plastic wrap, venting one corner. Microwave on High 5 to 7 minutes or until crisp-tender; drain well.
Meanwhile, in 2-quart saucepan, cook white sauce mix and 1 1/2 cups milk as directed on package. Remove from heat. Stir in cheese. Pour over zucchini mixture in baking dish; stir gently to coat. In small bowl, mix melted butter and bread crumbs. Sprinkle over zucchini mixture.
Bake 30 to 35 minutes or until bubbly and golden brown.