Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a baking sheet.

Place spaghetti squash cut-sides down on the prepared baking sheet. Bake in the preheated oven until a sharp knife can be inserted with only a little resistance, about 30 minutes. Remove squash from oven and set aside to cool enough to be easily handled.

Meanwhile, heat oil in a skillet over medium heat. Cook and stir onion in oil until tender. Add garlic; cook and stir until fragrant, 2 to 3 minutes. Stir in tomatoes and cook 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 cooked onion-tomato mixture, feta cheese, olives, and basil.

Serve warm.