Place about 1" water in lg pot. Cut off ends of swuash and add squash to pot. Bring to boil. Cover, cook until tender, about 30 mins.
Remove squash and let cool a little. In medium saucepan, over med-low heat, combine remaining ingredients, except 1 tbsp parmesan cheese.
Whisk until smooth. Cut squash in 1/2 lengthwise, discard seeds. Scrape the inside of squash w/ a fork, shredding it into spaghetti strands. Add strands to sauce, stir to mix and heat through. Sprinkle w/ remaining parmesan cheese.