Set your Instant Pot to sauté and melt the butter. Add the diced onion, garlic, and celery. Cook for 2-3 minutes until the onion softens, stirring in the garlic powder, oregano, thyme, rosemary, salt, and pepper.
Pour in the chicken broth and add the chicken breasts, ensuring they are covered. Lock the lid and pressure cook on high for 6 minutes.
Let the pressure release naturally for 5 minutes, then quick release any remaining pressure. Remove the chicken and shred it with two forks before returning it to the pot.
Mix the cornstarch with the warmed milk and stir it into the soup along with the shredded carrots and gnocchi. Set the Instant Pot to sauté mode and simmer for 2-3 minutes until the gnocchi float. Finally, stir in the spinach until it wilts. Serve hot, garnished with Parmesan cheese if desired.