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1
Preheat the oven to 350u00b0F.
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2
Chop tomatoes into quarters.
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3
Simmer the tomatoes with the olive oil: Combine the chopped tomatoes and olive oil in a large saucepan. Bring to a simmer. Cook until the tomatoes are soft and the peels begin to detach from the tomato flesh.
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4
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 into the pulp. Discard or compost the seeds and skins.
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5
Divide the tomato pulp between two large, rimmed baking sheets. You can also use a large roasting pan, but it will take longer to cook down that way.
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6
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 pans into and continue to bake.
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7
The paste is done when shiny and brick-colored, and it has reduced by more than half (3 to 4 hours). There shouldn't be any remaining water or moisture separating from the paste at this point. This will take 3 to 4 hours, though exact baking times will depend on the juiciness of your tomatoes.
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8
Divide finished paste into 4-ounce jars, leaving 3/4 inch headspace.
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9
Preserving - 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 nine months.
