Green Tomato Jam - Marmellata Di Pomodori Verdi - cooking recipe

Ingredients

5 lbs firm green tomatoes (unripe- I like San Marzano)
3 1/2 cups sugar
2 lemons, zest of
2/3 cup lemon juice

Preparation

Core the tomatoes, halve them, and dig out the seeds with a table knife. Cut the tomatoes into 3/4\" cubes (or smaller if you like).
Put a small plate in the freezer. You will use it later to test for doneness.
Place the tomatoes in a heavy 6-quart pot with the sugar, lemon zest, and lemon juice. Stir well, then bring to a boil over moderate heat. Reduce the heat to maintain a slow simmer and continue cooking until the tomatoes become translucent and the syrupy liquid thickens considerably, about 40 minutes, stirring occasionally to prevent the jam from sicking. (Note: towards the end keep an eye on it so it doesn't burn). When the jam reaches 215 degrees F (102 degrees C) on a candy thermometer, remove the chilled plate from the freezer and spoon a little jam onto it.
Return the plate to the freezer until the jam is cold. Test it for doneness (my note: prod it with your finger, if it seems to be developing a surface skin and solidifying it is ready), and if the jam is still too runny, continue cooking it. Otherwise remove pot from heat.
This will produce 2 pints or 4 half-pints worth of jam that can be processed via Water-Bath Canner. Fill jars to 1/2\" below the rim to allow for appropriate headspace. Process the jars in the canner for 15 minutes.