To make the filling, melt the butter in a saucepan over a medium heat, add the flour and cook for 1 minute to make a roux. Pour in the milk a little at a time, whisking constantly, until you have a thick sauce with no lumps. Add the spinach, nutmeg and a pinch of salt and cook for 1 more minute. Remove from the heat, fold in the remaining ingredients and leave to cool.
To make the pancake batter, mix the flour and milk in a bowl or jug, add the eggs with a pinch of salt and mix well with a whisk.
Brush a large (around 30cm) heavy frying pan with melted butter and set over a low-medium heat. Pour a small ladleful of batter into the pan and tilt it to make a thin pancake. Cook for about 2-4 minutes on each side. Repeat with the remaining batter to make 6 pancakes in total.
Heat the oven to 180°C fan/gas 6. Brush a high-sided baking sheet with melted butter. Put a spoonful of filling in the centre of each pancake. Fold over 2 edges to meet in the centre, then fold over the other 2 edges, making a parcel. Put the pancake parcels on the prepared baking sheet, sprinkle with the extra parmesan and cook in the hot oven for 12 minutes until crisp and golden on top.
