Why does white sauce go runny in a fish pie?

* Too much liquid in the filling: If you use a lot of watery vegetables like mushrooms or spinach, or your fish is very moist, it can release extra liquid into the sauce, making it thin.

* Insufficient thickening: The white sauce itself might not be thick enough. You might not have used enough flour or cornstarch for thickening, or you didn't cook the sauce long enough for the thickener to activate.

* Over-baking: If the fish pie is baked for too long or at too high a temperature, the sauce can cook out and become thin.

* Incorrect milk type: Using whole milk will give you a thicker sauce than using skimmed milk.

* Using cold milk: If the milk you use is cold, the sauce might not thicken as well.

Tips to prevent runny sauce:

* Wring out excess moisture: Squeeze out excess liquid from vegetables like mushrooms or spinach before adding them to the filling.

* Use a well-drained fish: If possible, use a fish that's already been drained of excess moisture, such as canned tuna or salmon.

* Make a thick white sauce: Ensure you use enough thickener and cook it for long enough, stirring frequently. You can even add a tablespoon of butter to the end for a richer texture.

* Bake at the correct temperature: Bake the fish pie at a moderate temperature, about 350-375°F (175-190°C).

* Don't over-bake: Check the pie frequently and remove it from the oven when the topping is golden brown and the filling is bubbly.

Fixing a runny sauce:

* Add more thickener: If the sauce is already baked, you can try adding more flour or cornstarch to thicken it. Be careful not to add too much, as it can make the sauce lumpy.

* Thicken with cornstarch slurry: Mix a tablespoon of cornstarch with 2 tablespoons of cold water, then stir it into the sauce and simmer until thickened.

* Add a sprinkle of flour: Sprinkle a tablespoon of flour over the surface of the sauce before baking, this will help absorb some of the excess moisture.

Remember, the best way to prevent runny sauce is to start with a thick, well-seasoned white sauce and a well-drained filling. With a little attention to detail, you can make a delicious and comforting fish pie.