Preheat the oven to 425°F.

Melt 2 Tbsp butter in a large skillet over medium-high heat.

Add in 1 finely diced onion and 3 chopped celery stalks.

Sauté for 2–3 minutes until softened.

Stir in 1 bag (8 oz) frozen mixed vegetables, 4 cups cooked shredded turkey, ½ tsp poultry seasoning, ¼ tsp thyme leaves, ½ tsp salt, ½ tsp pepper and 2 cups chicken broth.

Bring the mixture to a boil, then reduce the heat and simmer for 10 minutes.

In a separate bowl, whisk together 1 cup chicken broth (remaining broth) and ¼ cup cornstarch.

Whisk until completely smooth.

Pour the cornstarch mixture into the skillet.

Cook over medium-low heat for 4–6 minutes, stirring frequently, until the filling thickens.

Transfer the turkey mixture to a 9 x 13-inch baking dish.

Sprinkle 1 package (8 oz) dry stuffing mix evenly over the top of the casserole.

Drizzle ½ cup melted butter evenly over the stuffing mix.

Cover the dish with foil and bake for 30 minutes.

Remove the foil and continue baking for 15 minutes, until the topping is golden brown.

Remove from the oven and allow the casserole to rest for 10 minutes before serving.