Layer the strawberries and sugar in a large bowl. Cover and leave overnight.
The next day,scrape the strawberries and their juice into a large heavy pan. Add the lemon juice. Gradually bring to a boil over a low heat, stirring until the sugar is dissolved.
Boil steadily for 10-15 minutes or until the jam registers 220 on a sugar thermometer. Alternatively, test for setting by spooning a small amount onto a chilled saucer. Chill for 3 minutes, them push the jam with your finger: If wrinkles form on the surface, its ready. Cool for 10 minutes.
Stir the jam before pouring it into warm sterilized jars, filling them right to the top. Cover with waxed paper discs immediately, but do not seal with lids until the jam is completely cold.