Wash and dry the fruit with a paper towel.
Chop rhubarb into 2cm pieces, hull the strawberries then pit and halve the cherries.
Put all the fruit in a glass bowl and cover with the sugar and lemon juice and leave overnight together with the half lemon.
The next day, put 3 saucers in the freezer ready to test the jam’s setting point.
Clean 2 x 400gl glass jars and lids in your dishwasher (or pour boiling water into freshly washed jars and lids), then put in the oven upright at 140°C for 20 minutes and then turn off the oven leaving the jars to stay warm in there.
Pour the fruit/sugar mixture and half lemon skin into a preserving pan.
Remove a handful of smaller strawberries if you’d like some whole fruits in your jam.
Mash the fruit with a potato masher lightly and then add the whole strawberries back into the pan.
Heat the pan gently until all the sugar crystals are dissolved, stirring to check.
Bring the pan to a rolling boil and time for 10 minutes.
Spoon a few drops of jam onto a saucer, place in fridge and if it crinkles after a minutes when a finger is pushed through it, it is ready, if not carry on boiling for a couple of minutes (at a time) and repeat the test until ready.
Once ready, remove the lemon skin and stir the butter through the jam to remove any scum (or if you would prefer just remove the scum with a spoon)..
Use a small ladle to pour the jam into the warm jars and put the lids on immediately.