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. Saute onion in oil until tender. Add garlic, and saute 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 sauteed vegetables, feta cheese, olives, and basil. Serve warm.
Amount Per Serving Calories: 147 | Total Fat: 9.8g | Cholesterol: 17mg