In a skillet, soften the leek in the butter. Season with salt and pepper. Sprinkle with the flour and mix well. Add the remaining broth (125 ml / ½ cup), the ham, broccoli, milk and mustard. Bring to a boil. Off the heat, add the reserved broth and potatoes and mix well. Pour into a 28 x 20-cm (11 x 8-inch) baking dish.