1. Pierce squash several times with a fork; place in an 11 x 7-inch baking dish. Microwave, uncovered, at HIGH 6 minutes. Cut in half lengthwise; discard seeds. Place squash, cut sides up, in baking dish; add water. Cover tightly with heavy-duty plastic wrap, turning back 1 corner to allow steam to escape. Microwave at HIGH 5 minutes or until tender. Drain and cool 15 minutes.
2. While squash cools, melt butter in a large nonstick skillet over medium-high heat. Add onion and garlic; sauté 2 to 3 minutes or until onion is tender.
3. Using a fork, remove spaghetti-like strands from squash. Add strands (about 3 cups) to pan; cook 2 minutes or until thoroughly heated. Transfer to a bowl; add Parmesan cheese and remaining ingredients, and toss well.