Preheat oven to 350 degrees F (175 degrees C). Lightly grease a baking sheet. Place spaghetti squash cut sides down on the prepared baking sheet, and bake 30 minutes in the preheated oven, or until a sharp knife can be inserted with only a little resistance. Remove squash from oven, and set aside to cool enough to be easily handled. Meanwhile, heat oil in a skillet over medium heat. Sauté onion in oil until tender. Add garlic and sauté for 2 to 3 minutes. Stir in the tomatoes, and cook only until tomatoes are warm. Use a large spoon to scoop the stringy pulp from the squash, and place in a medium bowl. Toss with the sautéed vegetables, feta cheese, olives, and basil. Serve warm.