1.5kg fresh or frozen berries (we used raspberries and blackberries)
1.2kg white sugar
Juice of 1 lemon
1 vanilla pod, split
Wash chosen jars and lids in warm, soapy water and allow to drain.
Preheat oven to 140°C. Put a small saucer in fridge to chill. Put berries and sugar in a large bowl, then stir to combine and set aside for 30 minutes.
Meanwhile, line an oven tray with a damp wad of folded newspaper. Lay jars and lids on paper and put in oven for 20 minutes to sterilise.
Transfer berry mixture to a heavy-based saucepan and set over a medium heat. Add water and bring to boil, then add lemon juice. Scrape in seeds from vanilla pod, then add whole split pod to mixture. Boil, uncovered and without stirring, for 25-35 minutes or until jam gels when smeared on the chilled saucer, or reaches 105°C-106°C on a sugar thermometer.
When jam is at gelling stage, skim surface to remove any foam.
Pour jam into hot jars, filling to top as jam will shrink while cooling. When cold, seal jars and store in a cool, dry place.