Place the rhubarb in a glass bowl along with the vanilla seeds, vanilla pod, lemon zest, sugar and lemon juice, cover and leave overnight.
The next day place 2 saucers into the freezer.
Then place rhubarb mixture into a preserving pan or large heavy based pan.
Heat gently to dissolve all the sugar crystals, stirring with a wooden spoon (do not simmer).
Then bring the pan to a rolling boil (stirring with a wooden spoon continuously) and time for 7 minutes.
Put a few drops of jam on a chilled saucer and place in fridge for about 30 seconds.
Test for set by pushing your finger through the jam, if it’s ready it will wrinkle and be gel like.
If the jam is not ready keep boiling for another 2 minutes at a time and doing the plate test until it is.
Finally, remove the vanilla pod and use a jam funnel and or fruit ladle to fill your hot sterilised jars and seal with lids immediately.
Allow to cool before serving.
Store jars in a cool, dark place (can keep for years) and once open store in fridge and eat within 3 months.
This recipe makes about 740 ml of jam, ie just under 4 x 190 ml jars.
Before you start: Sterilise jars by washing in hot soapy water (or take straight from dishwasher), fill with boiling water, empty and then place in oven for 20 minutes at 140°C where you leave them until the jam is ready. Washed lids should be sterilised with boiling water and then left to drain (if they don’t dry in time I pop them in the oven once the jars are ready and just run the fan to dry them off for a few minutes).
If you’re pushed for time you can jump to step 3, soaking overnight helps retain fruit structure as well as letting the flavours develop.