Meanwhile, make spinach and artichoke mixture: Heat oil in a large pan over medium heat. Once hot, add onion and garlic and cook until beginning to soften, about 5 minutes.
Add in spinach and artichokes and cook until wilted, about 5 minutes. Season with ½ teaspoon salt and pepper and add cream cheese (or cashew cream) and cook for an additional 2 minutes.
Preheat the broiler on high.
Once spaghetti squash is cooked and cooled enough to handle scoop out the squash into a large bowl. Add parmesan cheese (if using), chicken (if using) and spinach artichoke mixture. Mix together gently until fully combined.
Either divide the mixture into the now empty squash shells and top with mozzarella cheese or place in an 8" x 8" oven safe casserole dish.
Place under the broiler until the cheese and melted and the mixture is completely heated through.