Step 1Heat the oven to 300 degrees. Cut the ripe tomatoes into a small dice; this promotes the most rapid cooking. Warm the olive oil in the largest skillet or casserole you own, add the tomatoes and salt and bring the tomatoes to a rapid boil. Cook the tomatoes for about 2 minutes, or until they are very soft. Immediately pass them through the finest plate of a food mill, pushing as much of the pulp through the sieve as you can. The puree should be devoid of seeds.
Step 2Lightly oil a baking sheet with olive oil. Spread the tomatoes on the pan in an even layer. Cook the tomatoes for about 5 hours, to evaporate the water from the paste. The tomato mixture will significantly reduce. Use a spatula to turn the paste over on itself periodically as water evaporates and you notice the surface darken. When most of the liquid is evaporated and the paste is quite thick, reduce the heat to 250 degrees and continue to evaporate the paste for about 2 more hours, or until it is thick, shiny and brick-colored.
Step 3Transfer the conserva to two clean half-pint glass canning jars. Tomato conserva holds for a long time stored in glass jars and topped with one-half inch of olive oil. As you use it, maintain this level of olive oil on top. Store the conserva in the refrigerator.