1. Put the strawberries in the pan, sprinkle over all of the sugar and place it into the fridge overnight.
2. Place the pan on a medium heat and let the sugar to dissolve. This will take roughly 6 minutes. Do not stir at this point as not to break up the fruit.Once dissolved bring to a boil and add the lemon juice. Stir briefly and boil rapidly for 8 minutes.
3. After 8 minutes spoon in the butter and stir in. Take off the heat and allow to cool for 20 minutes.
4. Spoon the strawberry jam into the jar. Cover it with a small square of parchment paper and seal tightly with an elastic band.
5. Finally, put the jar lid back on, closing it tightly.The strawberry jam can be kept in a cool, dry place for 2-3 weeks.