Sterilize glass jars - we'll need 3 small jars or 2 medium-sized jars. Google "how to sterilize glass jars" if you don't know how.
Now, place 1kg of frozen blueberries into a large pot. Place the pot on a LOW heat. Give the pot an occasional stir, and the blueberries will start to thaw and create some liquid.
Once there is some liquid in the pot, raise the heat to a MEDIUM. And bring the pot up to a boil.
Once the pot is boiling, set a timer for 10 minutes. Let it gently simmer away. Don't forget to gently stir the blueberries around once-in-a-while.
After 10 minutes, skim the bubbles (foam) off the surface.
Next, add in 1 Tablespoon of Kosher Salt (don't use fine dining salt!).
Next, add in 300 grams of Sugar.
Mix the sugar in - so it gets evenly incorporated.
Now reduce the heat to a LOW, and let the blueberries gently simmer for 20 minutes. Set a timer! (Don't forget to stir once-in-a-while during the 20 minutes...)
After 20 minutes is up, the blueberry mixture will still be runny. Add-in 3T of lemon juice and you'll feel it thicken. Give it a final stir. Then turn off the heat and take it off the stove. (The jam will continue to thicken in the residual heat).
Give it 5 minutes to cool down - as you may burn yourself if you try to bottle it immediately. After 5 minutes (while the jam is still somewhat hot), scoop it into your jars. It goes in easier while the jam is still hot.
Now, let the jars cool down - without the lid - for about half an hour.
Afterwards, twist on the lids and place in the refrigerator.
Once its cool, enjoy on some toast or greek yogurt!