Cut spaghetti squash in half and remove seeds and pulp. Place squash, cut sides down, onto a lightly greased baking sheet.
Bake at 350ºF for 30 minutes or until a sharp knife can be inserted with only slight resistance. Allow to cool slightly for handling.
Heat oil in a skillet over medium heat. Sauté onion until tender. Add garlic and sauté an additional 2 to 3 minutes. Stir in tomatoes and cook just until warm.
Using a large spoon, scoop out stringy pulp from squash; place in the bowl. Toss with sautéed vegetables, feta cheese, olives, and basil. Serve warm. Makes 6 servings.