Using a sharp knife, score the bottoms of each tomato in a cross, then plunge the tomato into boiling water for about 20 seconds. Remove and let cool, and when cool enough to handle, peel and roughly chop the tomatoes over a large pot, to catch the juices. (This is not a neat process. You might want to wear an apron.) Discard the skins, and place the tomato chunks into the pot.
Add the lemon juice to the pot, and add the lemon halves to the pot with the tomatoes. Heat the tomatoes over medium heat. Add the sugar, 1 cup at a time, stirring after each addition, and waiting about 10 minutes in between each addition. Lightly crush the tomatoes, or use a hand blender to puree slightly, and let cook over medium heat, stirring frequently, until the jam begins to jell and the entire mixture is bubbling like a swamp, being careful not to let it burn. Remove the lemon halves.
Ladle into hot canning jars with new lids, and process in a boiling water bath for 10 minutes.
Makes approximately 2 pints.
Recipe by The Domestic Front at http://www.thedomesticfront.com/preserving-summer-tomato-jam/