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.