Method

Sieve the flour into a large baking bowl and add the salt. Make a well in the center of the flour and add the eggs and beat well until smooth and lump free. Add half the milk and the tsp of butter and beat well. Add the remaining milk and stir.
Leave the batter to rest for 15 minutes.
Lightly grease a 9"/ 22cm pancake or frying pan with a little melted butter, heat until very hot, and add a ladle of batter to evenly and thinly coat the base of the pan. Cook until set and lightly golden. Flip over and cook on the other side for approx 30 seconds.
Remove the pancake from the pan, place on a sheet of kitchen paper and keep warm. Continue as above until all the batter is used up.

Filling
Place the flour and butter into a large saucepan. Over a low heat stir, the butter and flour until the butter melts and the flour is incorporated. Add a pinch of salt and pepper and continue stirring for 2 minutes.
Turn the heat up to medium and add the milk in one go and whisk furiously until all a smooth sauce is formed. Continue stirring until the sauce is thickened and glossy (about 5 minutes). If the sauce is thick, add a little more milk. The sauce should be thick but still runny. Remove from the heat.
Break the salmon fillet into large flakes and carefully stir into the sauce, followed by the prawns, mushrooms, and dill. Return to a gentle heat and warm.
Place a pancake onto the worktop, put a quarter of the seafood sauce mixture into the center of the pancake and spread in a long line from one end to the other. Fold the opposite edge of the pancake over the filling then roll the pancake into a thick sausage shape. Place the pancake into an approx 7" x 9"/17 x 22 cm ovenproof baking dish. Repeat with the remaining three pancakes.
Sprinkle the cheese over and bake in the preheated oven for 20 minutes or until the cheese is melted but not browned. Serve immediately with green vegetables or a salad.