Step 1Heat the oven to 350 degrees.
Step 2Pierce the skin of the squash in several places with a sharp knife or carving fork. Place the squash on a rimmed baking sheet and bake until tender when pierced with a fork, about 45 to 60 minutes. While the squash is baking make the sauce.
Step 3Heat the olive oil in a 10-inch skillet over medium heat. Swirl to coat the pan and cook the onion and garlic, stirring frequently, until softened and just beginning to brown, about 6 to 8 minutes. Add the tomatoes, adjust the heat so the sauce simmers slowly, and cook for 20 minutes. Add the zucchini and simmer 5 minutes longer. Add the parsley and salt and pepper to taste. Keep warm over very low heat.
Step 4When the squash is tender, halve lengthwise and scrape out the seeds with a spoon. Use a fork to scrape the flesh from the skin, and place the squash strands in a serving bowl. Toss with the butter, and season with salt and pepper. When ready to serve, spoon the tomato sauce over the top and sprinkle with the Parmesan cheese. (The squash can be prepared up to a day in advance. Refrigerate covered, and reheat in a 250-degree oven or in a microwave until hot. The sauce, without the addition of zucchini and parsley, can be made a day in advance. Bring to a simmer; then add the zucchini, parsley, salt and pepper as directed above.)