In a 12-inch ovenproof skillet, melt ¼ cup butter over medium heat. Add all mushrooms, onion, and garlic; cook, stirring occasionally, until vegetables are softened and lightly browned, about 10 minutes. Remove vegetables from skillet.
In same skillet, melt ¼ cup butter over medium heat. Whisk in flour; cook, whisking frequently, until lightly browned and smooth, about 5 minutes. Gradually whisk in broth and cream until smooth; cook, whisking constantly, until thick and bubbly, about 5 minutes. Stir in cream cheese until melted. Remove from heat; stir in vegetables, chicken, 1 tablespoon kosher salt, pepper, and celery salt.
In a medium microwave-safe bowl, melt remaining 2 tablespoons butter on low; stir in bread and remaining ¼ teaspoon kosher salt until well coated. Top chicken mixture with bread mixture.
Bake until golden brown and bubbly, about 25 minutes. Let stand for 10 minutes before serving. Garnish with parsley, if desired.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/creamy-chicken-mushroom-casserole/