Wash the strawberries and wipe with a kitchen paper. Remove the stem of the fruit with a knife or melon baller. Cut any large berries in half.
Place the strawberries in a large mixing bowl, add the jam sugar and mix together. Leave uncovered at room temperature overnight.
Put a saucer in the freezer. This will be used to test if the cooked jam has reached setting point.
Place the strawberry mixture into a large saucepan over low heat. Add in the lemon juice. Stir the mixtuure very gently until all of the sugar has dissolved. If any sugar granules remain on the side of the pan, use a damp pastry brush to brush the sugar granules away.
Once all the sugar has dissolved, turn up the heat to high and boil for about 6 minutes, or until it reaches the jam setting point (220 °F or 105 °C) on your thermometer. To test if the jam has set if you do not have a thermometer, spoon a little jam onto the cold saucer and leave for a minute. Then push you finger through the jam, if the jam wrinkles and does not flood the gap. it is ready. If the jam is still runny then return it to the heat for a few minutes and repeat the wrinkle test again. Repeat this step until the jam is ready. Remove any foamy scum at that has risen to the surface with a spoon and discard it. Mix in a tablespoon of butter into the finished jam until it melts.
To sterile your jars, pre-heat the oven to 160°C. Wash the jars and lids in hot soapy water and rinse, but do not dry them. Place the jars onto a baking tray and pop into the oven for 10 minutes. Leave the lids to soak in boiling water for 2 minutes,
Pour the jar into the sterilised jars. Place a wax disc on top of the jam and then cover with a lid. Leave to cool and then store in cool dry place.