Prepare the special sauce by mixing together the course ground mustard and boursin cheese
Lightly toast all 4 slices of bread. I do not butter my bread, but you certainly could.
Spread the special sauce on one side of all 4 slices of bread.
On two slices, layer 4 slices of roast beef each, followed by the arugula and 6 slices of granny smith apple. Top the sandwich with the remaining slice of bread, slice in ½ and enjoy!