Preheat oven to 400 F. Place squash open side down on baking pan. Bake for 40 minutes.
Heat oil in a large skillet. Add garlic and onions. Heat until onions are tender.
Add artichokes and spinach and cook until spinach is wilted.
Once squash is cooked, use a fork to loosen squash. Evenly mix in spinach/artichoke mixture to both sides of squash along with sour cream, cream cheese, 1/4 mozzarella cheese and 1/4 cup Parmesan cheese.
Top with remaining 1/4 cup of mozzarella cheese and 1/4 cup Parmesan cheese.
Bake at 350 F for about 10 minutes then broil for about 5 minutes.
Notes
If you would like this dish to be a bit more creamy, add another 2 tablespoons of cream cheese and 1/4 cup of sour cream.