Increase your oven temperature to 325 degrees Fahrenheit now. Melt three tablespoons of your butter in a medium saucepan. Set your stove to medium heat. Add your minced onion and celery to the pan. Sauté them for six to eight minutes until they look soft.

Add your minced garlic and all-purpose flour to the vegetables now. Cook the mixture for one minute while whisking constantly. Do not let the flour burn. Sprinkle in your garlic salt, kosher salt, and black pepper.

Pour your heavy cream into the saucepan very slowly. Keep whisking often to prevent any lumps. Bring the sauce to a gentle simmer. Cook it for about five minutes until it thickens nicely.

Spread your shredded chicken evenly across the bottom of your baking dish. Spread your sour cream directly on top of the chicken layer. Pour your warm cream sauce evenly over the sour cream layer.

Sprinkle your dried bread pieces thickly over the top of the sauce. Melt your remaining four tablespoons of butter in a small bowl. Mix the melted butter with your chicken broth. Pour this liquid evenly over the bread pieces.

Place the uncovered baking dish into your preheated oven now. Bake it for thirty minutes until the edges are bubbling. Turn on your broiler for the last three minutes. This creates a beautifully toasted and brown crust.