You’ll need to work on this marmalade on two days. On day one, carefully wash the citrus fruits under running water and dry them. Cut each fruit in half and squeeze it, collecting the juice in a bowl.
Strain the juice and put it aside, then collect the seeds left in the strainer in a cheesecloth.
Now cut each half of the fruit into four parts and remove the inner skin (the skin protecting each segment) with your hands. Collect this skin in the cheesecloth along with the seeds: both are rich in pectin and this will help your marmalade to set.
Slice the citrus peel as thinly as possible. Collect the juice and the sliced peel in a large pot and cover with 2 litres of water. Close the cheesecloth with a string to form a small bag and add it to the pot. Leave it overnight at room temperature.
The next day bring the citrus peel to a boil over medium heat, then lower to the minimum and simmer gently for about two hours. The content will reduce by half.
When the citrus peel is soft, carefully lift the cheesecloth bag out of the pot, place it in a strainer and press it well with a wooden spoon to have the pectin drip into the pot, then throw away the content. It has done its job.
Remove the pot from the heat and add the lemon juice and the sugar. Stir until the sugar has completely melted into the marmalade.
Put the pot back on the stove and bring to a boil, then simmer for about 20-25 minutes. If you have a pastry thermometer, the marmalade will be ready when it reaches 105°C (221°F). The marmalade will be still very liquid, it will thicken up once cooled down.
You can also check empirically whether the jam is ready or not with the saucer test. Pour a drop of marmalade onto a cold dish from the freezer. If it thickens and does not slip away when you tilt the saucer, the jam is ready to be poured into sterilized jars.
To sterilise the jars, you can boil the jars placed in a large pot and covered with water for about 20 minutes. Then turn off the heat and let the jars cool down completely before removing them.
They can be kept in the pantry for more than a year.