Using sauté setting – melt 2 tablespoons of butter in pot then add mushrooms and season with salt and pepper. Cook for about 7 minutes, until all juices are released. Transfer mushrooms with their juices to a bowl and set aside.
Without rinsing out pot, and continuing on sauté mode, add 2 tablespoons of butter, onions, and celery to pot. Cook for about 3 minutes, stirring frequently, until celery begins to soften. Then add vinegar, garlic, thyme, and bay leaf. Continue cooking for another minute.
Pass mushrooms through a strainer, returning their liquid to pot. Continue to keep the mushrooms separate to add them later. Add chicken stock to pot, along with chicken breasts, and cook for 7 minutes at high pressure.
Once cook time is complete, allow pressure to release naturally. Remove thyme sprigs and bay leaf and discard. Also transfer chicken breasts to a bowl.
Then, using two forks, pull the chicken apart into bite-size pieces.
Add cream cheese to pot and mix well until completely melted. Finally, return mushrooms and chicken to pot and mix well. Add the parsley and give one final mix. Serve immediately.