Croque-Monsieur French Toast

Ingredients
For the sandwich:
8 slices of brioche or white bread
4-6 slices of ham
4 slices of Gruyère or Swiss cheese
1/4 cup Dijon mustard (optional, for spreading)
For the custard:
4 large eggs
1 cup (240 ml) whole milk
1/4 tsp salt
1/4 tsp ground nutmeg (optional)
For cooking:
2 tbsp butter (for frying)
Optional toppings:
Béchamel sauce (recipe below)
Powdered sugar (for a sweet-savory twist)
Instructions