Spread the mustard over 4 bread slices and top each with 2 turkey slices, 2 ham slices, 1 Swiss slice and the remaining bread.
Press the sandwiches firmly and trim only loose overhanging meat so the edges sit flat.
Beat the eggs, milk and salt in a shallow dish and dip each sandwich quickly on both sides without soaking the center.
Melt the butter on a griddle over medium-low heat and cook the sandwiches for 3 to 4 minutes per side until golden and hot in the center.
Dust lightly with powdered sugar, rest for 2 minutes and cut each sandwich diagonally.
