To make the filling: in a saucepan, stir the mustard into vegetable stock until dissolved. Bring to a simmer over medium-high heat, then add cream and butter.
Reduce heat to medium and simmer until reduced by half. Stir in crab and prawns; keep warm over low heat.
To make the sauce: In a small saucepan, warm cream and mustard over medium heat. Once hot, whisk in the grated cheese gradually to make a smooth sauce. Season with nutmeg, salt and pepper. Keep warm over low heat.
To make the pancakes: In a bowl, whisk eggs and cream together until smooth. Season with salt and pepper.
Heat a 20cm non-stick frypan over medium heat, and brush lightly oil. Pour a little of the egg mixture into hot pan, and swirl to make a thin, even layer.
Cook until egg sets then flip over and cook for a few seconds more until firm on the other side. Repeat process making 6 thin omelettes in total.
Spoon some of the seafood filling into the lower half of each. Roll up and serve with the warm cheese sauce.