Preheat the oven to 350°F. Arrange 2 racks to divide the oven into thirds and heat to 350°F.

Chop tomatoes into quarters. Quarter the tomatoes.

Simmer the tomatoes with the olive oil. Place the olive oil in a large pot over medium-high heat until shimmering. Add the tomatoes and cook until soft and the peels begin to detach from the tomato flesh.

Pass the tomatoes through a food mill. Push the warm tomatoes through a food mill, sieve, or chinois to separate the tomato pulp from the seeds and skins. Stir the sea salt and citric acid or lemon juice into the pulp. Discard or compost the seeds and skins.

Place the pulp on 2 baking sheets. Divide the tomato pulp between 2 large, rimmed baking sheets. You can also use a large roasting pan, but it will take longer to cook down that way.

Bake the tomato pulp until reduced to a paste. Place the baking sheets in the oven. Check the tomatoes every half hour, stirring the paste and switching the position of the baking sheets so that they reduce evenly. Over time, the paste will start to reduce to the point where it doesn’t fill the baking sheet any more. At this point, combine the contents of the two baking sheets into one and continue to bake.

Bake until reduced by more than half. The paste is done when shiny, brick-colored, and reduced by more than half, 3 to 4 hours, though exact baking times will depend on the juiciness of your tomatoes. There shouldn’t be any remaining water or moisture separating from the paste at this point.

Transfer the paste into jars. Divide the finished paste into 4-ounce jars, leaving ¾-inch headspace.

Preserving Option 1 — Process the tomato paste in a hot water bath. Apply lids and rings and process in a boiling water bath for 15 minutes. Keep in a cool, dark place for up to 1 year. After opening, refrigerate for up to 1 week.

Preserving Option 2 - Refrigerate or freeze. If you don’t want to process the paste, you can refrigerate or freeze it instead. Scrape finished paste into clean half or quarter pint jars. Top each jar with a layer of olive oil and place in either the refrigerator or the freezer. As long as you keep it well-covered with olive oil and ensure that you only use a very clean spoon to remove it from the jar, it will keep in the fridge for 3 to 4 weeks. Frozen, it will keep for up to 9 months.