Preheat oven to 400 degrees. Lightly grease a baking sheet with olive oil. Place spaghetti squash cut sides down on the prepared baking sheet. Bake 40-45 minutes, or until a sharp knife can be inserted with little resistance. Remove squash from oven, and set aside to cool enough to be easily handled.
Meanwhile, heat 2 tablespoons of olive oil in a skillet over medium heat. Sauté onion until tender. Add garlic, and sauté for 2 to 3 minutes. Stir in tomatoes, and cook only until tomatoes are warm.
When the squashes are cool enough to handle, scrape the flesh of each squash with a fork into strands, and place into a large bowl. Toss with the sautéed vegetables, feta cheese, olives, and basil. Season with salt and pepper to taste. Serve warm.