Sterilise your jam jars - either wash them in warm soapy water and then dry them out in the oven at 100C - or wash them (on their own!) in the dishwasher on the highest setting! Leave them to the side.
Prep your fruit - I took the tops off the strawberries and quartered them depending on size, took the red currants off of their stalks, and washed all four fruits
In a LARGE pan add the fruit - if you think the fruit is still a bit large then mush it down slightly with a potato masher, but this is up to you
Add the jam sugar and lemon juice and mix with a wooden spoon
Turn the heat on and warm through slowly at first so that the mixture has time to mix together and the sugar have time to settle into the mixture and stir occasionally
After about 5 minutes, turn the heat up high and boil the mixture. This is when I added my jam thermometer - it clips onto the side of the pan so is easy to use, and has a marker at 105C of where jam sets at!
Boil your mixture for 5 minutes (this is how long mine took) If you don't have a jam thermometer that's okay, put a plate in the fridge so its cold.
Once the 5 minutes of constant boiling is up then put a small blob on the plate. After a few seconds it should have semi set so it wrinkles when you run your finger through it
If it reaches 105C on the thermometer then its also ready! If it doesn't set slightly then boil it for another 2 minutes or so
At the same time, put your sterilised jam jars into the oven at about 100C so they are warm when the jam is poured into them!
Towards the end/when its finished boiling, skim off all the sediment that forms (you'll know what I mean!) and carefully pour into the warmed sterilised jam jars - it might spill everywhere so if you have a funnel then great, if not use a ladle
Seal the jars with the sterilised jam lids when still warm and leave to cool completely
Once cooled, these will store for 3 months at room temperature in a cupboard (can be longer, such as up to 6 months, but mine NEVER lasts that long!)
Once a jar is reopened it will last for 2 weeks in the fridge!
ENJOY!
Notes
I think the combination of fruit that I used is perfect - as its peak berry season they produce such a lovely flavour, but you can use a different ratio of berries if you wish just make sure it sticks at 1.2kg of fruit!
You can also use granulated sugar to make the jam, but the jam sugar has the added pectin ingredient which helps the jam massively and makes it last much longer in the cupboard!