Place quinoa and water in a medium-sized pot and cook about 15 minutes, until all the water has been absorbed and the grains are translucent. Remove from heat and let stand about 10 minutes. Fluff grains gently with a fork, and set aside.
Slice spaghetti squash in half lengthwise and discard any seeds. Bake cut-side down in a 400° oven about 45 minutes or until fork-tender. Let cool briefly, then scrape out the flesh using a fork and transfer strands to a large bowl.
[NON-YOM TOV MICROWAVE OPTION: The squash is delicious when I poke holes in it with a knife and place it on paper towels. I microwave it until it softens, about 20 minutes. Then I easily cut it open, scoop out the seeds, and fork it to strand it. I am a really busy person and I find that the microwave really is easier and safer for me -- it turns itself off !]
Add cooked quinoa and mix together briefly. Stir in pasta sauce, and sprinkle with shredded cheese, if using. Return to oven and bake at 350° for about 10 minutes to allow casserole to heat through and for cheese to melt. Serves 6-8.