Prepare the béchamel by melting the butter in a saucepan, stirring in the flour, and gradually adding the milk, stirring all the while. When the mixture begins to thicken, remove from heat and season with mustard, nutmeg and salt. Spread béchamel on bread slices, place ham on half the slices, top the ham with grated Gruyère and truffle slivers, and add the second slice topped with béchamel. Add a few more slivers of truffle and top with more grated Gruyère. Slip in the oven at 425°F for 1o minutes or so until cheese is brown and bubbling.