Start by melting some butter in a saucepan. Add the flour and whisk. Add the milk gradually and continue to whisk. Finally, add the gruyere and the salt to taste.
Spread the truffle butter on both sides of the bread slices. Add a slice of Swiss cheese, some grated gruyere, freshly shaved truffles, more grated gruyere, and a drizzle of truffle honey. Close the sandwich with a second slice of bread.
Toast until golden brown, and serve with the truffle mornay sauce, grated gruyère, and freshly shaved truffle.