combine flour, Italian seasoning, and garlic powder in a medium bowl. Season with salt and black pepper, as desired, and stir to combine.
Place chicken breasts in a one-gallon plastic freezer bag and pour seasoned flour mixture on top. Seal bag and turn several times until chicken is evenly coated.
Select “Sauté” function on Instant Pot® and set to high. Add one tablespoon olive oil. Once hot, remove chicken from bag and shake off excess flour mixture. Add enough chicken to cover bottom without overcrowding. Work in batches, if necessary.
Brown chicken on each side, approximately 3 minutes per side. Remove and repeat with remaining chicken, adding more olive oil as necessary for each batch.
Add butter to Instant Pot®. Once melted, add Portobello mushrooms and red onion. Cook, stirring frequently, until mushrooms turn brown and the onions soften, approximately 5-6 minutes.
Add Marsala wine and chicken broth and deglaze bottom of the cooking container by gently scraping up the brown bits with a plastic spatula.
Turn Instant Pot® off and add browned chicken to the pot and cover. Lock the lid and set the pressure valve to “Sealing.” Press the “Manual” button and adjust setting to “high.” Set cook time to 10 minutes.
When cook time is finished, allow pressure to release naturally for 10 minutes, and then do a quick release for any remaining pressure. Turn the unit off and remove lid. Transfer chicken to a serving platter and rest for 5 minutes.
To make the sauce, temper heavy cream with approximately 2 tablespoons of the hot liquid from Instant Pot®. Once tempered, add heavy cream, cream cheese, and chopped parsley to the pot. Season with salt and black pepper, to taste, and stir to combine.
Top each chicken breast with some of the creamy mushroom sauce and serve immediately. Enjoy!