Wash the tomatoes then cut them in half (if using cherry size or smaller) or slice them to an even thickness.
Dehydrate or oven-dry the tomatoes. If using a dehydrator, set to 60C (140F). If using an oven, set to the lowest possible temperature, and crack the door open a little if necessary (you want them to dry out, not cook). Depending on the thickness and whether you leave the seeds and pulp in, expect them to take from 6-14 hours to dehydrate. Whichever method you use, make sure the tomatoes are evenly spaced on your racks or baking trays, as maximum airflow is very important when drying food.
Once the tomatoes have dried to a pliable but leathery consistency, remove them from the heat source to cool fully. Put the tomatoes into a bowl.
Juice the lemon then pour the juice over the tomatoes. Mix to coat.
Sprinkle the salt over the tomatoes and mix well.
Decant the tomatoes into a jar, discarding any remaining lemon juice.
Fill the jar with oil, making all the tomatoes are submerged.
Notes
Note that the quantities listed here are quite variable. Adjust the oil, salt and lemon according to the amount of tomatoes you have.