To make the filling, melt the butter in a large saucepan. Add the leeks and cook over a medium heat for 10–15 mins or until the leeks are soft and starting to caramelise. Do not rush this process as it brings out the sweetness of the leeks. Add the wine or stock and bubble until reduced by half.
Add the cream, mustard and half the cheese. Stir for 2 mins or until the cheese has melted. Remove from the heat and add the remaining cheese. Season with salt and pepper to taste.
To make the pancakes, sieve the flour into a large bowl and stir in a pinch of salt. Make a well in the centre and crack in the egg. Whisk, gradually adding the milk and incorporating the flour from the edges. Stir in 1 tablespoon of the melted butter.
Place a 20cm non-stick frying pan over a medium heat. Carefully wipe the pan with some of the remaining (melted) butter using a sheet of kitchen paper.
Add 3–4 tbsp of batter to the pan and swirl to cover it evenly. Cook for 1–2 mins or until lightly browned underneath. Flip or turn over carefully and cook for a further minute. Slide onto a plate, cover with baking paper and cover loosely with foil or a dry tea towel to keep warm.
Repeat until all the batter is used up.
Spoon the filling onto the pancakes and fold into triangles. Serve with a fresh green salad.
