Prepare the filling by stirring Dijon mustard into chicken stock in a pan until dissolved. Bring to a simmer over medium-high heat, then add 60ml cream and 2 tablespoons butter. Reduce heat to medium, and simmer until reduced by half, then stir in crab and prawns; keep warm over low heat.
Prepare the sauce by warming 60ml cream and 1 teaspoon mustard over medium heat. Once hot, whisk in the grated cheese, then season to taste with nutmeg, salt and pepper. Keep warm over low heat.
Whisk eggs, 60ml cream, salt and pepper together until smooth. Heat a 20cm / 8-inch non-stick pan over medium heat, and lightly oil. Pour a little of the egg mixture into hot pan, and swirl to make a thin, even layer of egg. Cook until firmed, then flip and cook for a few seconds more to firm the other side.
Spoon some of the seafood filling into the lower half of each omelette. Roll up and serve with Cheddar sauce.