Remove the stem from each cherry, and press a chopstick or reusable straw down through the stem hole. Gently but firmly press the chopstick down into the pit and push it out of the cherry. Then cut them in half or roughly chop them, and place them in a large bowl.
Add the juice of one lemon and two thick lemon peel slices to the bowl. Stir in the sugar, cover with cling film and refrigerate for 2 hours or up until 12 hours.
Transfer the cherry mixture into a deep, heavy-bottomed dutch oven or stockpot, and cook over low heat for about 45 minutes, stirring frequently.
Once the jam has reached a foamy rolling boil, start checking if it's ready with a kitchen thermometer or using the "wrinkle test". The jam is ready when it reaches 220°F / 105°C or when you spoon a little jam on a chilled plate and it starts to wrinkle after one minute. If it still liquidy, continue to boil the jam for 5 minutes then retest until it's done.
Use a spoon to skim any scum that has risen to the surface and discard this. Remove the pan from the heat, and discard the lemon peels.
Allow the jam to cool, then you can leave it chunky, or blend it for a few seconds until it reaches your desired consistency.
Pour the jam into small jars or containers. Seal and let them cool to room temperature, then store the jam in the refrigerator for up to two weeks.
You need 500g (17oz) of pitted cherries - about 1,2 kg (2-½ pounds) of unpitted cherries.
This recipe makes enough for a full medium-sized jar.
Depending on the degree of sweetness of the fruit, you can increase the sugar amount. I used mostly sweet black cherries, so if you're using mostly bright red cherries, increase the sugar amount by ⅓ US cup or 70 grams.
You can marinate the cherries for 2 hours up to overnight. A longer marinating time will increase the release of pectin from the lemon.
A wide pan – big enough so that the fruit comes no more than halfway up the side – is better, as the jam will reach the setting point more quickly.
Use a wooden spoon to stir your jam as it cooks, avoid metal utensils.
The easiest way to test your jam is by using a kitchen thermometer, the setting point for jam is 220°F / 105°C.
You should start checking the temperature when the jam has reached a rolling, foamy boil.
If you don't have a kitchen thermometer, try the “wrinkle test.” About 10 minutes before cooking the jam, place a heat-proof plate in the freezer. When it's time to test your jam, spoon a little onto the cold plate. If the surface of the jam wrinkles when you nudge it with your finger, the jam is ready.