Rinse the tomatoes and cut out the core, removing any blemishes. Halve or quarter the tomatoes to easily fit into the jar.
Pre-heat the water in the canner and bring it to a boil.
Into each clean (750 ml to 1 L wide mouth) sterilized jar, put 1 teaspoon of salt and 1 teaspoon of sugar.
Add tomatoes skin side up, straight into the jar.
Push each tomato down to squash out the juice and continue adding tomatoes, until the jar is full. You may need the handle end of a wooden spoon to push down the tomatoes further into the jar.
Wipe the rim of the jar with a clean towel and place the lid on, leaving about 1/2 to 3/4 an inch gap, between the tomatoes and the very top of the jar. This headspace is important for the jars to seal.
Remove trapped air bubbles with a knife between the food and the jar. Continue around the jar and allow all the air bubbles to escape.
Carefully place the jars on the canning rack and let them sit for a few minutes. This is to make sure the jar is not cold when lowering them into the boiling water.
Make sure that the jars are fully submerged by at least an inch over the top.
Once the water is back to a boil with the jars fully submerged, then start a timer for 45 minutes and reduce the heat to medium low. The water needs to stay at a low boil.
Carefully loft the rack and remove the jars from the boiling water.
Allow the jars to cool for at least 12 hours before testing the seals. Just press down on the lid with your finger. They should be slightly concave with no movement up and down.
If a jar is not sealed properly then keep it in the fridge for up to a week or use it right away.