Pierce squash with knife 5 or 6 times. Arrange squash on paper towel in microwave and microwave at HIGH 15 minutes or until soft, turning over halfway through cooking. Cut in half; remove and discard seeds. With fork, gently scrape squash to pull apart spaghetti-like strands.
Meanwhile, heat olive oil in a large nonstick skillet over medium-high heat and cook onion, red pepper and yellow squash 5 minutes, stirring occasionally, or until vegetables are tender. Add sauce and simmer 4 minutes, stirring occasionally.
Serve sauce mixture over spaghetti squash and garnish, if desired, with grated Parmesan cheese.