Trim off the tough green ends of the leeks, then cut a line down the length of each one to wash out all of the soil trapped amongst the layers. Remove any rotten outer layers, and rinse well shaking out excess water. Slice the leek into 1/2″ pieces, discarding the hairy ends.
Wash the orange and grate off the rind with a fine grater. Reserve the rind for later, and juice the orange.
In a large, heavy bottomed pan, melt some butter to cover the base on a medium heat. Throw in all of the leeks and sweat for around 20 mins until the leeks are really soft. Pour in the juice and turn the heat up to evaporate the liquid, stirring frequently. Finally, add the sugar and stir until the crystals have dissolved, and the leeks are a thick, almost paste-like consistency. Turn off the heat, and season with salt to taste.
Transfer to a sterilised jar, or glass if you are serving straight away. Leek jam is the perfect accompaniment for any meat, fish, root vegetable, or egg dish. It will keep in the fridge for a week.