1. In a large, deep baking pan, combine eggs and buttermilk. Whisk together completely.
2. Add sugar and salt to egg and stir.
3. Lay challah slices flat in the egg mixture, and allow to sit for 30 minutes
4. Flip challah and allow other side of bread to soak for 30 minutes.
5. Heat 1/2 tablespoon of butter over medium heat in a large frying pan.
6. Put two soaked pieces of challah in the pan and cook until one side is golden, the flip and cook second side.
7. Spread 1 tablespoon of mustard on golden side of challah.
8. Add 2 ounces Gruyere on top of mustard.
9. Add several slices of ham on top of cheese.
10. Place second piece of challah on top of ham to make sandwich.
11. When the bottom slice is golden, flip entire sandwich over to finish the uncooked side.
12. Repeat with remaining ingredients to make a second sandwich.