01 - Preheat the oven to 375°F and lightly grease a 9x13-inch baking dish.
02 - Melt 3 tablespoons butter in a large skillet over medium heat. Add chopped onion, diced carrots, and celery. Sauté for 5 to 6 minutes until softened.
03 - Sprinkle the flour over the vegetables and cook, stirring continuously, for 1 minute to eliminate raw flour taste.
04 - Gradually whisk in the broth followed by the milk. Stir constantly and cook until the sauce thickens, about 3 to 4 minutes.
05 - Incorporate garlic powder, dried thyme, black pepper, and salt into the sauce. Mix well.
06 - Add the cooked turkey and frozen peas to the sauce. Stir to combine and simmer for 2 minutes to meld flavors.
07 - Transfer the turkey mixture evenly into the prepared baking dish.
08 - Sprinkle shredded cheddar cheese evenly over the top of the mixture.
09 - In a small bowl, mix breadcrumbs with melted butter. Evenly sprinkle over the cheese layer.
10 - Bake uncovered for 25 to 30 minutes until the topping is golden brown and bubbling.
11 - Optionally garnish with chopped fresh parsley. Serve hot.