Rub the orange peel and then squeeze out the juice.
Put all ingredients in a large saucepan and bring to the boil. Then let it simmer over a low heat (can take up to two hours). Stir well, especially towards the end, as otherwise the mixture can easily burn.
At the end of the boiling time, lightly puree with a hand blender so that the mixture remains lumpy.
Fill into twist-off glasses while still hot, close immediately and let cool upside down.