Preheat oven to 180 degrees C. Lightly grease a baking tray.
Place spaghetti squash cut sides down on the prepared baking tray, 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 frying pan over medium heat. Fry onion in oil until soft, about 5 minutes. Add garlic and fry further 2 to 3 minutes. Stir in the tomatoes, and cook only until tomatoes are warmed through.
Use a large spoon to scoop the stringy pulp from the squash, and place in a medium bowl. Toss with the vegetables, feta, olives, and basil. Serve warm.