Preheat oven to 350°. Spray a 2- to 3-quart baking dish with cooking spray.
In a large skillet, melt butter over medium heat. Add mushrooms, onion, and garlic; cook, stirring occasionally, until tender, 3 to 4 minutes. Add flour; cook for 2 minutes, stirring constantly. Gradually whisk in broth. Cook, stirring constantly, until thickened, 4 to 5 minutes. Remove from heat; whisk in milk and cream cheese until smooth.
Stir in cooked chicken, Broccolini, poppy seeds, celery salt, and pepper. Transfer to prepared dish.
In a small bowl, stir together sourdough crumbles and melted butter. Sprinkle over casserole.
Bake until bubbly, 25 to 30 minutes.
Let stand for 10 minutes before serving.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/poppy-seed-chicken-casserole/