Place the ziti noodles in inner pot of instant pot.
Add water, salt, garlic and onion powder, gently mix in.
Place the butter (sliced) on top of the pasta, close the instant pot with lid in sealing position.
Cook in high pressure for 1 minute.
Once the cooking is done, wait for a natural pressure release for 10 minutes, then after that gradually do a controlled manual pressure release.
Once the pressure is all released, carefully open the lid of instant pot pressure cooker (do not stir the cooked pasta at this point).
Add olive oil, shredded cheese, milk and heavy cream, now gently stir in with the pasta so all the ingredients are mixed with the pasta to form a creamy sauce.
Finally, add the fresh spinach leaves and marinara sauce, gently mix in to combine with the pasta. Enjoy the instant pot baked ziti hot for best results!
As the pasta cools down, the sauce will thicken naturally.
You may store the cooked pasta in air tight containers and store in refrigerator for another couple of days, so you can reheat and use as needed!