• Prepare the berries. Throw the berries into a large saucepan and cut any of the larger strawberries and blackberries in half. 
  500 g frozen berries
• Defrost. Add the sugar and lemon juice.  Stir in and allow to defrost, for a few hours or overnight. The sugar will draw out the juice and you will have a lovely dark purple syrup in the pan. 

  You may think this looks like a lot of sugar. Don’t be tempted to reduce the sugar, as the jam will not set and will spoil.
  500 g sugar, 2 tbs lemon juice
• Cook the fruit. Heat gently and allow the jam mixture to simmer but not boil, until all the sugar has dissolved. With the quantities suggested, this will take about 5–10 minutes. You can add a splash more water if you think you need to, but this will make the next step take longer.

  While you are cooking the jam on the stove top, heat your jam jars and lids in the oven on a low heat so that they will not crack when you fill them. I heat a Pyrex style jug too, so that I can use it to fill the jars without making a mess.

  If using the wrinkle test for setting, put a side plate or two in the freezer.
• Bring up to a rolling boil. Once the sugar has dissolved Slowly bring the berry jam up to boiling point and continue to cook until it reaches setting point (105°C / 221°F).

  Stir frequently, and scrape the bottom of the pan as you cook so that none of the fruit catches and burns. This will take about 10 minutes, depending on the size and shape of your pan.
• Setting the jam. As you approach setting point, the jam will thicken and you will see a change in the bubbles in the jam as they become larger and more defined. This will happen after a few minutes of rolling boil. Start to test for setting.

Testing for set

• Temperature: Use a jam thermometer, and check to see that the jam has reached 105°C/221°F.  Do not use this as the only method of checking for set.
• Wrinkle: Spoon some hot jam onto a chilled plate and see if it wrinkles ahead of your finger when pushed through the jam.
  
• Flake test: Allow the jam to run off a spoon with the bowl held vertically. When a flake of jam remains attached to the lip of the spoon, it has reached the setting point.
  

After reaching setting point

• Allow to cool. Once the jam has reached the setting point, turn off the heat and allow it to cool for about 10 minutes. This allows the jam to thicken slightly, so that the fruit pieces don't all float to the top in the jars.

  Stir and check that the fruit remains evenly distributed. If the fruit starts to float to the top, leave for another 5 minutes before checking again.
• Transfer your jam into the warm, sterilized jars. Spoon the jam into the hot jars and seal the lids.

  Wipe away any drips and seal the jars with the lids, making sure they are secure. Leave them to cool. Before you put them away, check that they have sealed properly.

  There is a little dimple in the lids of modern jam jars that will pop down as the jam cools, creating a secure seal. If not, you should be able to pop it down with a gentle pressure. If it does not stay depressed, the jar is not well sealed.

  A jar that won’t seal properly should be treated as if it has already been opened. Keep it in the fridge once cool and treat it as if already open, eating it within a few weeks.

  Label and start enjoying fresh, three ingredient frozen mixed berry jam!