Place the spaghetti squash on a large cutting board. Using a large knife, slice the squash in half lengthwise. Then, using a spoon, scoop out the seeds.
Add 1 cup of water and the trivet to the Instant Pot. Place both squash halves on top of the trivet. Lock the lid and cook for 2 minutes at high pressure.
Once the cooking time is complete, quick-release the pressure and remove the lid. Remove the squash and set aside to cool.
Give the pot a quick rinse and wipe. Switch to the sauté setting and add the olive oil, diced onions, and ¼ teaspoon of salt. Cook for 2-3 minutes, stirring frequently, until the onions are soft and translucent.
Once the onions are soft and translucent, add the minced garlic and sliced olives. Cook for 1 minute, stirring frequently.
Add the baby spinach and the remaining salt, and continue cooking until wilted.
Turn off the heat and add the fresh basil, diced tomatoes, and half of the feta cheese. Mix well until the tomatoes are warm.
Using a fork, scrape out the flesh of the squash and discard the shell.
Finally, add the remaining crumbled feta cheese and mix one last time. Serve warm and enjoy!