The make the parsley sauce, melt the butter in a large saucepan. Add the flour, cook for 2-3 minutes then slowly add the milk to the pan, whisking continually, until you have a thick white sauce.
Add the parsley and cook for a further five minutes. Season with salt and freshly ground black pepper.
Meanwhile, place the fish on large baking trays lined with greaseproof paper. Season with salt and freshly ground black pepper and put in the oven for 7-10 minutes or until the fish is cooked through. (You may have to do this in batches.)
To serve, place the fish on the plate and pour over the parsley sauce.