Preheat your oven to 350°F (175°C).

Melt 2 tablespoons of butter in a skillet over medium heat. Add diced onion, carrots, celery, and minced garlic, sauté for 5-7 minutes until softened.

Sprinkle in 1/4 cup of flour over the vegetables, stir well, and gradually whisk in 2 cups of chicken stock and 1 cup of heavy cream. Cook for another 5-8 minutes until thickened.

Stir in cooked chicken, frozen peas, thyme, salt, and pepper, mixing thoroughly before transferring to a baking dish.

In a bowl, combine 1 cup of panko breadcrumbs, 2 tablespoons of melted butter, and 1/4 cup of grated Parmesan. Mix well.

Pour the chicken mixture into the baking dish and spread evenly. Top with breadcrumb mixture.

Bake for 30 minutes until bubbly and golden brown.

Let cool for 5-10 minutes before serving.