Not a Croque, Croque Monsieur

Finely chop the leeks and give them a good wash in warm water. Melt butter in a saucepan and cook the leeks until soft. Add some flour in to make a roux. Add warm milk unto you have a thick sauce. Mix in gruyere cheese, Worcestershire sauce, grain mustard and season. Toast the bread on one side. Spread with the cheese sauce, add some ham and place the other toasted piece of bread toasted side down. Gently fry the sandwich in butter until golden and crisp. Serve sliced with extra cheese sauce for dipping