Melt butter in a large skillet over medium heat. Add mushrooms and cook, stirring often, for 3 minutes. Add onion and cook for 5 minutes more.
Stir together broth and cornstarch; add to skillet and whisk until boiling. Cook, stirring with a whisk until sauce is thickened, about 3 minutes. Stir in frozen vegetables. Cook until heated through, about 2 minutes. Stir in turkey and parsley and cook for about 2 minutes, until hot. Serve stew over noodles, toast or biscuits.