Traditionally, sliced ham and gruyere make up this elegant version of a grilled cheese, a staple in Parisian cafes. Some versions boost the creamy component with bechamel and broil it to a bubbling knife-and-fork sandwich. We nod to Italy with ours, obviously, cooking it “in carrozza”-style (in a carriage), subbing in mortadella and fontina and hitting it with a little powdered sugar at the end. This is one grilled cheese you’ll never forget.
Whisk the eggs, milk and salt in a shallow baking dish. Lay out the bread slices. Spread 1½ teaspoons of the Dijon on each of 4 of the slices. Shingle 4 mortadella slices on the mustard. Press 1/4 cup of the fontina on the mortadella. Cover each sandwich with a second slice of bread.
Heat a 12-inch nonstick skillet over medium heat for 1 minute. Melt 1 tablespoon of the butter. Submerge both sides of two of the sandwiches into the egg mixture and place in the skillet. Cook until well-browned, 3 to 4 minutes. Flip and cook the other side 3 to 4 more minutes. Remove to plates (or keep warm in a 200 degree oven). Repeat with the remaining tablespoon of butter and the other 2 sandwiches. Sift some confectioners’ sugar over the sandwiches before serving.