Steps

with the butter, flour, whole milk and 12g of gruyère make a Mornay Sauce

layer the sandwich with bread, ham, most of the mornay sauce, half the remaining gruyère, mustard, bread

toast in a frying pan over medium heat for the outsides of the bread are toasted and the contents are warned through

spread the remaining mornay sauce over the top of one side and top with the remaining gruyère

broil the sandwich until the top is golden brown