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RINSE the strawberries and remove the stems, the green part. To preserve the jam in the jars, they need to be really clean. Bacteria inside the jars will ruin the strawberry jam.
Therefore it's recommended to bring a large pan with water to a boil and cook the jars for a few minutes. Place them upside down on top of a clean kitchen towel and that's it.
CHOP the strawberries into quarters and add them to the medium-sized pan along with the jelly sugar and juice of 1/2 a lemon. Turn the heat to medium-high and boil the strawberries about 5 minutes.
Be careful, it will start to bubble and it's very hot. You can use a fork to mash the strawberries a little bit if you don't like pieces of strawberry in your jam. Carefully DIVIDE the jam over the clean preserving jars.
Place the (metal) lid on and place the jars upside down on the kitchen towel. Let them cool off completely like this, this will create a vacuum so the jars can be stored for a very long time! Enjoy!