Place spaghetti squash on large cutting board. Using a large knife, slice squash in half lengthwise. Then, using a spoon, scoop out seeds.
Add 1 cup water to pot, along with trivet. Place both squash halves on top of trivet. Lock lid and cook for 2 minutes at high pressure. Once cook time is complete, quick-release pressure and remove lid. Transfer squash to large mixing bowl.
Give pot a quick rinse and wipe. Switch to sauté setting…
Add oil, onions, and 1/4 teaspoon of salt – cook for 2-3 minutes, stirring frequently.
Meanwhile… using a fork, scrape out flesh of squash. Discard shell.
Once onions are soft and translucent, add garlic and olives. Cook for for 1 minutes, stirring frequently. Add spinach and continue cooking until wilted. Then turn off heat and add basil, tomatoes, and salt. Mix well until tomatoes are warm.
Return squash to pot and mix well. Finally, add feta and mix one last time.