Let's cook.
Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Melt 2 tablespoons of butter in a large skillet over medium heat. Add the sliced leeks and cook, stirring occasionally, until softened, about 5-7 minutes.
Stir in the cooked chicken, heavy cream, Dijon mustard, salt, and black pepper. Bring the mixture to a gentle simmer, then remove from heat.
Pour the chicken and leek mixture into the prepared baking dish, spreading it evenly.
In a small bowl, combine the Panko breadcrumbs and grated Parmesan cheese. Sprinkle this mixture evenly over the chicken and leek gratin.
Bake for 25-30 minutes, or until the gratin is bubbly and the breadcrumb topping is golden brown and crispy.
Let the gratin rest for 5 minutes before serving to allow the sauce to set slightly.
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