Cut the spaghetti squash in half lengthwise and scrape out the seeds with a spoon. Drizzle 2 teaspoons extra-virgin in each of the squash halves and sprinkle with salt. Place them flesh-side down on a baking sheet. Roast until you can fluff the squash easily with a fork, 45 to 50 minutes.
Flip the squash halves and fluff the "spaghetti" strands with a fork, then season with salt. Pour 2/3 cup sauce over each of the squash halves, and top each with 1/2 cup shredded mozzarella. Bake until the cheese is browned and bubbly, another 25 minutes. (You can flip the oven to the broiler at the last minute for extra cheese browning!)