For the pancakes, place the flour and salt into a bowl. Make a well in the centre and break the eggs into it.
Gradually whisk in the milk and 75ml/2¾fl oz water until smooth. Whisk in the melted butter until well combined.
Heat a little oil in a 18cm/7in frying pan until hot, then reduce the heat to medium. Spoon about 60ml/2fl oz of batter into the pan, and swirl it around to get an even covering.
After about 30 seconds, turn the pancake over using a fish slice, or flip the pancake in a swift motion. Cook for a further 25-30 seconds, remove the pancake from the pan and set aside. Cook the remaining batter and place greaseproof paper in between each pancake. (You should make about eight pancakes.)
For the sauce, melt the butter in a saucepan, then add the flour and stir for 2-3 minutes minutes. Gradually whisk in the milk. Season to taste with salt and freshly ground black pepper. Add the nutmeg and lemon zest, and whisk until well combined. Cook the sauce for another five minutes, then fold in the fish, take off heat. Add the tarragon and tomatoes.
Remove the pan from the heat and spoon a little seafood mixture into each pancake, roll them up and place into a baking dish.
Meanwhile preheat the oven to 180C/350F/Gas4
For the topping, mix the parmesan, breadcrumbs and butter together in a bowl until well combined. Sprinkle the mixture on top of the pancakes and bake for 8-10 minutes, or until golden-brown.