Make béchamel sauce (see instructions below).
Turn oven on to grill. Brush the bread with melted butter and place in oven, butter-side up, to toast one side (watch that the bread doesn’t burn).
Heat oven to 220oC bake. Turn bread over. Spread two slices with a little béchamel, followed by most of the gruyere and all of the ham. Add a second slice of bread on top to form two sandwiches. Spread béchamel over the top and sprinkle with remaining cheese. Bake for 10-15 minutes, until golden brown.
Place cream, milk, onion, garlic and bay leaves in a saucepan and bring to the boil. Turn off heat and leave to infuse for 10 minutes. Strain through a sieve and reserve the liquid.
Return empty saucepan to a medium-low heat and add butter to melt. Stir in flour and
cook for 30 seconds. Gradually add the strained milk mixture and continue cooking until
nice and thick. Stir in mustard and nutmeg. Season with salt and pepper to taste.