Preheat the oven to 200°F. Place a wire rack on a baking sheet and set it in the oven to heat. In a large bowl, whisk together the eggs, milk, and salt until fully combined.
Spread a thin layer of mustard on one side of each bread slice. On half of the slices, layer cheese, turkey, then more cheese. Top each with the remaining slices of bread to form sandwiches.
In a large nonstick skillet over medium-low heat, melt 2 tablespoons of butter. Working with two sandwiches at a time, dip each sandwich into the egg mixture to coat, then cook in the skillet, turning once, until golden and the cheese is melted—about 4 to 6 minutes per side. Transfer the cooked sandwiches to the wire rack in the oven to keep warm. Repeat the process with the remaining 2 tablespoons of butter and the remaining sandwiches.
Serve the sandwiches on plates. Lightly dust with confectioners’ sugar and offer raspberry preserves on the side.