In a large pot, combine the chicken broth, carrots, celery, onions, mushrooms, thyme, bay leaf, and rosemary. Bring to a boil over medium-high heat, then reduce the heat, and simmer simmer until the vegetables are tender. Add the corn, green beans, poultry seasoning, salt, and black pepper.
In a small bowl, whisk to dissolve the cornstarch in ¼ cup cold water. When completely dissolved, add it to the pot and stir to combine. Let the mixture return to a simmer and you should notice the cornstarch thickening the liquid within about 2 minutes. If your stew doesn't thicken enough to your liking, add more cornstarch (dissolved in cold water; generally the ratio of cornstarch to water is 1:2).
Now that the mixture has thickened, stir in the cooked, chopped chicken. Let the mixture cook for about 5 minutes for flavors to blend. Stir in the parsley.