Step 1

Preheat oven to 350 degrees F. Arrange bread cubes in a single layer on a large baking sheet. Bake until evenly toasted and crisped, about 15 minutes. Let cool for about 10 minutes.

Step 2

Meanwhile, whisk broth and cornstarch in a small saucepan until smooth. Bring to a boil over medium-high heat, stirring often. Cook, stirring constantly, until thickened and glossy, about 1 minute. Pour into a large heatproof bowl; let cool for about 10 minutes.

Step 3

Heat 1 tablespoon butter in a medium skillet over medium-high heat. Add onion, carrot and celery; cook, stirring often, until softened, 5 to 8 minutes. Add to the broth mixture in the bowl along with chicken, sour cream, thyme, poultry seasoning, salt and pepper; stir until thoroughly combined.

Step 4

Measure 1 1/2 cups of the toasted bread cubes into a small bowl, and set aside. Stir the remaining bread cubes into the chicken mixture. Coat a 2-quart baking dish with cooking spray; spoon the chicken mixture into the baking dish.

Step 5

Place the remaining 3 tablespoons butter in a small microwaveable bowl; microwave on High until melted, about 30 seconds. Add to the reserved bread cubes and toss to coat. Sprinkle evenly over the chicken mixture.

Step 6

Bake until golden and bubbly, about 30 minutes. Let cool for about 10 minutes. Serve.