Preheat oven to 375 F. Lightly grease a baking sheet.
Place spaghetti squash with cut sides down on the prepared baking sheet, and make for 30 minutes in the preheated oven, or until a knife can be inserted with only a little resistance. Remove from oven and set aside to cool enough to be easily handled.
Heat olive oil in a skillet over medium heat. Cook and stir onion until tender. Add garlic, cook and stir for 2 to 3 minutes. Stir in tomatoes and cook until tomatoes are warmed through.
Use a large spoon to scoop the pulp from squash and place in large bowl. Toss with the stir from skillet, feta cheese, olives and parsley. Serve warm.