Lower the heat and add the lemon slice, allow to simmer for about 45 mins on low heat. (the lemon prevents the sugar from caramelizing).Turn the heat off after 45 mins and remove the lemon.Allow to cool for 5 mins then strain into a clean jar.Once it has cooled down it will thicken.After it has cooled completely cover with the lid and store in the cupboard it can last for months.