Preheat the oven to 450 degrees F. Cut the squash in half and scrape out seeds. Place flesh-side up on an oven tray. Cover with foil, and roast for 30 to 40 minutes until fully cooked. Remove from the oven and let rest for 10 minutes. Using a fork, pull all the shredded flesh from the skin and set aside.

Puree the tomatoes until very smooth in a blender or food processor.

Heat 3 tablespoons olive oil in a large skillet over medium heat. Add the onion and cook until soft, about 5 minutes. Stir in the garlic, anchovies, red pepper flakes, and capers and fry for 2-3 minutes. Add the tomato puree, olives, black pepper, and 2 tablespoons basil. Bring to a boil and then turn heat to low and simmer, stirring occasionally, until thickened and juice from tomatoes is boiled off, about 15 minutes.

Portion out spaghetti squash and top with sauce. Garnish with remaining basil and parmesan.