How to Make It
Spread half of the tomatoes in a large saucepan. Cover with 2/3 cup of sugar, the lemon slices and another 2/3 cup of sugar. Spread the remaining half of the tomatoes and 2/3 cup of sugar on top. Let stand without stirring until most of the sugar has dissolved, about 1 hour.
Stir the tomatoes. Bring to a boil over moderate heat, stirring frequently. Reduce the heat to moderately low and cook, stirring occasionally, until the jam cooks down to 3 cups, about 1 hour. Let cool, then discard the lemon slices.