Chicken and Tater Tot Casserole

Name: Chicken and Tater Tot Casserole
Ingredients: 4 tablespoons unsalted butter; 2 carrots, cut into 1/2-inch thick rounds; 2 stalks celery, cut into 1/4-inch half-moons; 1 onion, chopped; Kosher salt and freshly ground black pepper; 1/4 cup all-purpose flour; 2 cups chicken broth; 1 cup whole milk; 4 cups shredded cooked chicken; 3 cups frozen potato tots, such as Tater Tots, thawed; 1 teaspoon fresh thyme leaves