Preheat oven to 350º. Cut squash lengthwise and remove seeds from each half. Place facedown on rimmed cookie sheet, sprayed with cooking oil. Bake for 30 minutes.
In the meantime, heat oil in a large saucepan. Add onion and garlic and sauté until tender. Season with salt and pepper. Stir in tomato and sauté until softened.
When squash is finished, pull out of the oven and let cool for a few minutes. When cool enough to handle, flip squash over and scrape insides of each half with a fork. Little spaghetti-like strands will easily come off the squash.
Stir squash with sautéed onion and tomato mix. Add olives. Sprinkle with chopped parsley.
Makes a great, healthy side dish for grilled chicken or seared shrimp!