Chicken Pot Pie Casserole

Arrange a rack in center of oven; preheat to 375º. In a large skillet over medium-high heat, melt butter. Add onion, carrots, celery, thyme, 1/2 tsp. salt, and 1/2 tsp. pepper and cook, stirring often, until softened, 5 to 7 minutes.

Add flour and stir to coat vegetables. Cook, stirring, until flour is distributed, about 2 minutes. Add milk, broth, mustard, garlic powder, and ½ tsp. salt, whisking to combine. Increase heat to high and bring to a boil. Immediately reduce heat to medium and simmer, stirring often, until thickened, 3 to 5 minutes. Fold in chicken and peas; season with salt and pepper, if needed.

Transfer mixture to a 13" x 9" baking dish. Arrange biscuits on top of filling; season all over with salt and pepper.

Bake casserole until filling is bubbling and biscuits are golden and risen, 22 to 25 minutes. Transfer to a wire rack and let cool slightly.