Try Justine Schofield’s take on this French classic.
Lightly grill one side of each of the sourdough slices.
Smear the mustard on the untoasted sides of the bread.
Assemble the sandwich using the ham and some gruyère and béchamel.
Close the sandwich and pour some more béchamel on top and sprinkle a little gruyère cheese.
Place under the grill for approximately 2 minutes, making sure it doesn't burn.