Start by washing both jars and lids with warm soapy water and then rinse them well. Dry the jars with a tea towel before arranging them upright in a roasting tin. Place the tin in a 160°C oven for 10 minutes.
Slide an empty plate on the refrigerator rack, to use later as a base for testing the jam’s setting point.
Blanch the lemon zest by soaking it in boiling water for 2 minutes, press the juice and set aside.
After you’ve pitted all the cherries, transfer them to a pan and cook them on medium to high heat for about 10 minutes. By then the juices run and the fruit is soft. To add a bit of consistency mash some of the cherries with the back of the spoon.
Reduce the heat and combine the sugar into the fruit. Stir to dilute.
When the sugar is melted, strain the lemon zest and incorporate it together with the juice into the jam. Bring to a rapid boil until setting point is reached. Skim the froth with a slotted spoon.
To check setting point, drop a teaspoon of jam on the cold plate from the fridge. Let it to cool for a minute or so, and then run your finger through it, if it leaves a trace then the jam is ready.
Allow the jam in the pan for 10 minutes before ladling it into the warm and dry jars. Fill to the very top and cover with lids.