Mix the flour, egg, sugar, milk, parsley, and salt into a cream-consistency batter.
Heat olive oil in a frying pan and cook two thin pancakes until golden-brown on both sides.
Melt butter in a saucepan and stir in flour for two minutes over low heat.
Gradually whisk in the milk and cream until smooth.
Boil and simmer the sauce until thickened, then stir in mustard, chives, and seasoning.
Brush leeks with oil and cook in a hot griddle pan until tender and chargrilled.
Fill pancakes with leeks and crumbled Stilton, roll tightly, and fry until crisp.
Serve the pancakes topped with the sauce and remaining leek garnish.