Add the butter to a large 6-8 quart soup pot and set over medium heat. Once melted add the onions, celery, and garlic.
Sauté for 2-3 minutes, then add in the mushrooms, rosemary, and thyme. Sauté until the mushrooms soften and cook down.
Deglaze the pot with dry sherry. Then add in the whole chicken breasts, chicken broth, 1 teaspoon salt, and 1/4 teaspoon black pepper. Bring to a boil. Then lower the heat and simmer for 15 minutes, or until the chicken breasts have cooked though.
When the chicken is fully cooked, remove the breasts with tongs, and chop into bite-size pieces.
Whisk the cornstarch into the heavy cream, making sure there are no clumps. Then whisk the heavy cream into the soup pot and allow it to simmer and thicken. Stir the chopped chicken back into the soup and add the parsley. Taste, then salt and pepper as needed.