Turn the sauté function on your Instant Pot. Once it reads HOT, add in the butter.
Once butter is melted, add in the onion and cook for about 3 minutes.
Add in the garlic and cook an additional 30 seconds.
Stir in the chicken broth, garlic powder, pepper, salt, and mushrooms, bouillon cubes, and chicken.
Turn off the sauté function and place lid securely on top. Ensure the valve is in sealing position.
Cook manual, high pressure, for 6 minutes, followed by a natural pressure release for 10 minutes.
Once time is up, do a quick release to remove any additional pressure.
Once pin has dropped, remove lid carefully.
Carefully remove the chicken to a plate and cover/set aside.
Turn the Instant Pot back to saute function.
Warm the half and half in the microwave for 45 seconds.
Whisk the flour into the warmed half and half until smooth and combined.
Whisk the mixture into the Instant Pot as this will help thicken the sauce.
Add in the Parmesan cheese and spinach, seasoning with additional salt/pepper if desired.
Turn off the Instant Pot and serve chicken with sauce either over pasta, mashed potatoes, or on its own! Enjoy!