Transfer the ham to a warm serving plate, and rest for 20 minutes. Meanwhile make the sauce. Finely chop the parsley (by hand or using a food processor, which makes light work of this).
Ham, gruyère, dijon mustard, and béchamel sauce are sandwiched between two slices of bread, coated in even more béchamel and cheese, and broiled until the top is melty and blistered.