For the cheese, melt the butter in a saucepan over medium heat, add the flour and stir continuously for a minute.
Using a whisk, pour in a quarter of the milk, whisking vigorously to prevent lumps. Keep whisking and add in the rest of the milk. Whisk until the sauce comes to the boil and thickens.
Season with the nutmeg, salt and pepper. Remove off the heat and stir in parmesan.
Brush four of the tortillas with mustard and spread over a tablespoon of the cheese sauce leaving a 1 cm border. Add ham and then sprinkle over the Gruyere cheese. Sandwich the last 4 tortillas on top.
Brush the quesadilla with oil and heat a large pan over a medium heat. Working with one quesadilla at a time, cook for 1-2 minute until golden and the cheese begins to melt.
Carefully flip and cook for a further 1-2 minutes. Repeat with the remaining.
