Cut squash in half lengthwise; remove and discard seeds. Place, cut side down, in ungreased 13x9-inch (3-quart) microwave-safe dish. Add 1/4 cup water to dish. Cover with microwave-safe plastic wrap. Microwave on HIGH for 8 to 13 minutes or until tender.
Meanwhile, spray large nonstick skillet with nonstick cooking spray. Add mushrooms, onion and garlic; cook and stir over medium heat for 3 to 5 minutes or until tender. Add sauce; bring to a boil. Simmer 2 to 3 minutes or until thoroughly heated.
While sauce is simmering, use fork to remove spaghetti-like strands of cooked squash; place in serving bowl. Serve sauce over squash.