Crack the egg into a shallow dish, add one teaspoon of water and beat with a fork.
Preheat a medium sized pan over medium heat.
Meanwhile, assemble the sandwich. Spread one of the slices of bread with jam and top with turkey or ham (or both!) and Swiss cheese. Spread the Dijon on the other slice of bread and close the sandwich with the mustard side down. Dip both sides of the sandwich in the egg mixture.
Add the butter to the preheated pan and use a spatula to coat the surface evenly. Transfer the sandwich to the pan and cook until golden then flip and repeat on the second side.
Use a fine mesh sieve to sprinkle powdered sugar on top and enjoy!