Chicken Pot Pie Casserole

Preheat oven to 375º with rack in the center. In a large skillet, melt butter over medium-high heat. Add carrots, celery, onion, thyme, and ½ teaspoon salt, and cook, stirring often, until softened, 5 to 7 minutes. Add flour, and stir to coat vegetables. Cook for 2 minutes more, then add milk, chicken broth, mustard, garlic powder, and remaining 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 . Transfer mixture to a 13"-x-9" baking dish.  Arrange biscuits on top of filling. Sprinkle all over with salt and pepper. Bake until filling is bubbling and biscuits are golden and risen, 17 to 20 minutes. Let cool 5 minutes before serving.