How do you thicken fish pie?

To thicken fish pie, you can use a roux made from equal parts butter and flour, cooking them together before gradually adding milk or fish stock to create a creamy sauce. Alternatively, you can incorporate cornstarch or arrowroot mixed with a little cold water, adding it to the sauce and cooking until it thickens. Adding mashed potatoes or breadcrumbs as a topping can also contribute to the overall thickness of the dish.