Thaw the strawberries out of the freezer. Take them in a bowl of food processor and process it until it is smooth. Transfer it in a large pot and add sugar on top and stir. Turn the heat on to high and bring it to the boil. Turn the heat down to medium. Stirring occasionally, cook it for 15 minutes. Add lemon juice and stir. Boil for another 5 minutes over medium heat. Turn the heat off and let it cool for ten minutes. Keep it in the refrigerator.