4 tbsp. unsalted butter
2 carrots, cut into 1/2″ thick rounds
2 stalks celery, cut into 1/4″ half-moons
1 onion, chopped
1 tsp. kosher salt
3/4 tsp. 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 tater tots, thawed
1 tsp. fresh thyme leaves
Preheat the oven to 450 degrees F.
Melt the butter in a large heavy pot over medium-high heat until hot. Add the carrots, celery and onions and season with salt and pepper. Cook, stirring occasionally, until the vegetables are golden, about 12 minutes.
Stir in the flour, and then whisk in the broth and milk and bring to a boil, whisking. Boil until slightly thickened, about 3 minutes. Stir in the chicken. Transfer to a 2-quart baking dish.
Pulse the tots in a food processor with the thyme until medium ground. Scatter over the top of the casserole and bake until the filling is bubbling and the topping is golden and crisp, 20 to 25 minutes. Let stand 10 minutes before serving.