Step 1

If you're using plum tomatoes, cut them in half lengthwise. If you're using round tomatoes, cut them into quarters.

Step 2

Remove the seeds with your fingers. Place all the tomatoes in an 8-quart stainless steel pot and bring to a boil, stirring occasionally, until the tomatoes release their juices. Boil briskly for 30 minutes until the tomatoes soften and the juices reduce.

Step 3

Pass the tomatoes through a food mill fitted with a fine disk to remove the skins and any remaining seeds.

Step 4

Return the tomato purée to the same pot and place it over high heat. Stir in the salt, reduce the heat to medium-ish, and simmer, stirring frequently, until the purée has reduced to about 1 quart (4 cups), 45 to 55 minutes. You'll need to turn the heat down as the purée thickens to prevent it from furiously bubbling and splattering.

Step 5

Lightly slick a 12-by-17-inch rimmed nonaluminum baking sheet with oil. Using a rubber spatula, spread the thick tomato purée in an even layer. It should cover the entire baking sheet.

Step 6

Preheat the oven to 200ºF (93ºC) and turn on the convection fan if you have one. Position a rack in the center.

Step 7

Place the baking sheet in the oven and bake for 30 minutes.

Step 8

Remove from the heat (keep the oven on) and stir the purée with the rubber spatula so that it dries evenly and doesn’t form a crust. Respread the purée with the spatula into a rectangle about 1/8 inch thick. Be fanatical about spreading it evenly; if any part is too thin, it may burn. Because of evaporation, the purée will no longer cover the baking sheet. With a paper towel, remove any bits of tomato that cling to the edges or exposed bottom of the baking sheet.

Step 9

Return the baking sheet to the oven and continue baking until the tomato purée is no longer saucelike but very thick, stiff, and a little sticky, about 3 more hours. Every 20 minutes, stir and carefully respread the purée as before. The rectangle will become progressively smaller as the remaining water evaporates. Taste and, if desired, add more salt.

Step 10

Let the tomato paste cool to room temperature.

Step 11

Use a spoon and move the paste to a clean jar, tamping it down to make sure there are no air pockets. Level the surface with the back of the spoon. Cover the surface completely with olive oil so that the paste isn't exposed. Screw the lid on the jar and refrigerate. It will keep in the refrigerator for at least a year.

Step 12

When using this homemade tomato paste, dole it out by the teaspoon to add depth to dishes. Always wait to salt the dish until after you've added the tomato paste as it will bring quite a lot of concentrated saltiness. Each time you scoop out some tomato paste from your jar, level the surface of the paste and top it with more oil so the remaining tomato paste is completely submerged.