- Put a small ceramic plate in the freezer. You will use this this to test the jam later.
- Safely prepare you jars for canning.
- Chop the tomatoes and place them in a large Dutch oven (optionally see crock pot procedure at the bottom) with the sugar and vinegar. Stir everything together, then turn the heat to medium and simmer until the tomatoes begin to break down and are soft, about 15 minutes. Blend the tomatoes with an immersion blender until you have a smooth puree. Lower the heat and continue simmering the jam, stirring frequently, until the liquid has reduced and the mixture is thick and spreadable. This could take anywhere from 40 minutes to over an hour, depending on how juicy your tomatoes are. As the mixture thinks, stir more often and watch carefully to avoid scorching.
- When the jam has cooked down and is thickened, spoon a little jam onto the frozen plate. Leave it to set for a minute, tilt the plate. If the jam stays put, or only runs a little bit, it’s ready. Also, run a finger through the jam on the plate if the two sides stay separate and don’t run back together it is another sign it is ready.
- When the jam has met the set test, remove it from the heat. Fill your canning jars with jam, leaving a 1/2-inch head space. Wipe the rimes of the jars with a damp paper towel to clean up any sticky spills. Dry the lids with a clean paper towel and place on the jars. Screw on the bands tightly. Use good canning procedure.
- You can also make this in the slow cooker. To do this, place all the ingredients in the cooker and cook uncovered for 12–14 hours until set. Puree the tomatoes when they are soft.