Place the bread slices on a cutting board and spread 4 slices with dijon and mayonnaise. Top with turkey and Gruyere. Place the remaining bread slice on top to form 4 sandwiches.
In a shallow bowl, whisk together the egg and milk. Season with a dash of kosher salt and freshly ground black pepper.
Melt the butter in a large griddle over medium heat.
Dip each sandwich on both sides into the egg mixture and place on the griddle. Cook sandwiches about 3 minutes on each side or until they are lightly browned and the cheese has melted. Slice sandwiches on the diagonal and serve.
We used Udi’s White Sandwich bread for the bread in this recipe.