Start by sterilising your jars. This recipe produces two small jars, or one medium-sized jar. My favoured method is to wash the jars and lids then pop them all in a large saucepan of cold water. Whack it on the heat and bring to the boil. Once bubbling, set the timer for 10 minutes. Afterwards, turn off the heat and scoop out the jars with some clean tongs. Leave them on a clean tea towel to air-dry.
Meanwhile, rinse your blackberries in a couple of changes of cold water to remove any unwanted friends. Peel and dice your apples. Weigh them as you go to make sure you have 150g. Tip the fruits, sugar, lemon zest and juice into a sturdy saucepan or jam pan.
Set the heat to high. The berries will release their juices and the sugar will dissolve. Watch and stir occasionally to prevent the sugar from burning.
It will start to bubble and froth. Scoop away the dusky pink foam with a spoon as it will dull the jam.
Add the bay leaf. Reduce the heat to medium-low and cook the apples until squidgey-soft and collapsing. This should take 10 minutes. Stir to check that the apples have completely reduced.
Pop a saucer in the freezer. Yes, this is a thing, I’m not crazy.
With a little jug of water at your side, increase the heat. This will be quick so keep the stove clear and utensils at the ready. Keep stirring the jam and add splashes of water if it appears dry. Spoon a blob of jam onto the cold saucer. If it flows around the plate then cook for another few minutes. If it is loose but thick, and you can wipe your finger through without the two sides flowing together then it is ready.
Turn off the heat and remove the bay leaf. Leave to cool slightly then spoon into the jars. Screw on the lid nice and tight, and turn the jar upside down to seal the vacuum of the lid. Store in the fridge overnight until cool then spread thickly on toast.