1. In a medium-sized bowl, whisk together the ingredients for the French toast batter.
2. Heat a little butter or oil in a large non-stick pan. Dip a slice of bread into the French toast batter, making sure to coat both sides; place it in the hot pan. Cook for around 2 minutes, or until the bread is golden.
3. Repeat the process with the second slice of bread.
4. To assemble, spread the Dijon mustard on both slices of bread. Place the slices of ham on one slice of bread, then top with the parmesan. Add more chives, to taste, then close up the sandwich with the second slice of bread.
5. Briefly heat the sandwich in the hot pan to melt the parmesan. Remove from heat, cut in two, and enjoy.