In a large skillet heat the canola oil over medium heat.
Place the spaghetti squash cut side down in the pan and allow to brown (approximately 5 minutes).
Remove and continue the same process with the second squash.
*Note - This step is optional, but the extra browning of the squash will add more flavor to the final soup.
Place the browned squash in a microwave-safe dish and cover with plastic wrap OR preferably a paper plate, or paper towels.
Microwave until the squash becomes tender and strands will form when the interior of the squash is scraped with a fork.
Set aside to cool.
Meanwhile, in the same skillet over medium/high heat brown and crumble the Italian sausage (approximately 7-10 minutes).
Add the onion, zucchini, and garlic.
Cook, stirring occasionally until the sausage is no longer pink in the center.
When the spaghetti squash is cool enough to handle, scrape the strands of the squash into a large stockpot.
Add the sausage mixture, oregano, basil, tomatoes, red pepper flakes, salt, black pepper, and chicken stock. Stir well.
Bring the soup to a simmer and cook over low for 20-30 minutes.
Stir in the butter and apple cider vinegar.
Remove from the heat.
Add additional seasoning to taste, if desired.
To serve, spoon the soup into bowls and top with Parmesan cheese.