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The night before you make your jam, layer the berries, peaches and sugar in a very large bowl. Cover and set aside at room temperature.
The next morning, give everything a good stir and set aside again until you’re ready to start cooking. Put a small saucer in the freezer.
Tip the raspberries and peaches, scraping out all the juices and any undissolved sugar, into a preserving pan (or use a large, wide-based pan – the wider and more open the pan, the faster the jam will be ready without the fruit cooking to mush). Stir in the lemon juice and cook over a very low heat until any remaining sugar is dissolved.
Bring the mixture to the boil, then simmer for 5 mins. Turn off the heat and spoon a little of the hot jam onto the chilled saucer. Once cooled, push the jam with your finger – if it wrinkles a little, it has reached setting point. If it’s still too runny to wrinkle, return the pan to the heat and boil in 2-min stages, removing the pan from the heat each time you repeat the saucer test, until ready.
Skim off any scum, then stir in the butter, if using, to dissolve any remaining scum. Leave for 15 mins, then ladle into sterilised jars (see below). Will keep for up to 6 months in a cool, dark place.