01 - Preheat the oven to 375°F.
02 - In a large skillet, melt 3 tbsp butter over medium heat. Add the onions, carrots, and celery. Sauté for 5 minutes until softened.
03 - Stir in the flour and cook for 1 minute. Gradually whisk in the broth and milk, stirring constantly until the mixture thickens, about 3–4 minutes.
04 - Add salt, pepper, and thyme. Stir in the turkey and peas. Simmer for 2 minutes, then remove from heat.
05 - Transfer the mixture into a greased 9x13-inch baking dish.
06 - In a small bowl, combine breadcrumbs and melted butter.
07 - Sprinkle the casserole with cheddar cheese, then top with the breadcrumb mixture.
08 - Bake for 30–35 minutes, or until the top is golden and the casserole is bubbling.
09 - Let stand for 5 minutes before serving.