- Preheat oven to 180c and the grill to high.
- Split croissants lengthways and toast under grill, cut side up.
- Melt the butter in a saucepan over medium low heat. Add the flour and cook, stirring for 1 minute.
- Add the milk, reduce heat to low and whisk for 5/6 minutes until thick and smooth.
- Add the mustard, lemon zest and three quarters of the cheese, then season well.
- Lay the ham slices on the croissant bases and spread with most of the cheese sauce.
- Sandwich the croissant tops and spread each top with a little of the remaining sauce.
- Sprinkle with the remaining 25g cheese and bake for 6 minutes or until bubbling and golden.