- 1 medium onion, roughly chopped
- 4 stalks celery with leaves, roughly chopped
- 4 cloves garlic
- 7 pounds plum tomatoes
- Kosher salt
- 1 1/8 teaspoons citric acid (also called sour salt), for preserving
- 18 fresh basil leaves
- Six 16-ounce canning jars with lids and bands
- Clean kitchen towels
- 2 large stockpots
- Jar lifter or canning tongs
- Food mill
- Sterilize the jars: Wash the jars, lids and bands in hot soapy water and rinse well. Place a folded clean kitchen towel or canning rack in the bottom of a stockpot (this keeps the jars from rattling); fill about halfway with water. Add the jars, making sure they’re submerged. Bring to a boil and boil 10 minutes; remove with a jar lifter and place on a clean towel. Turn off the heat; reserve the pot of water. Put the lids and bands in a saucepan of simmering water until ready to use (do not boil).
- Cook the tomatoes: Bring another stockpot of water to a boil. Add the onion, celery and garlic and cook until just tender, about 8 minutes. Add the tomatoes (in batches, if necessary) and cook until they split and rise to the top, about 5 minutes. Strain the vegetables; discard the celery stalks but keep the leaves for flavor.
- Puree the tomatoes: Set a food mill over a large bowl. Working in batches, puree the tomatoes and vegetables through the mill, scraping the bottom occasionally with a fork. Stir in 2 tablespoons salt, then the citric acid-this ensures a safe acidity level for canning.
- Fill the jars: Remove the lids and bands from the hot water with tongs and place on a clean towel to dry. Put 3 basil leaves in each sterilized jar (make sure your hands are clean). Use a ladle or funnel to fill the jars with the tomato puree to about 1/2 inch from the top. Wipe the rims of the jars with a damp paper towel, then screw on the lids (do not overtighten).
- Process the jars: Bring the stockpot of water to a simmer (keep the towel in the pot). Lower the filled jars into the pot, bring to a boil and process 45 minutes. Use the jar lifter to remove the jars and transfer to a towel; let sit overnight. Don’t overfill the jars-you need to leave some space.
- Check the seals: Press the tops of the jars: The lids should not pop up and down. Store in a cool dark place for up to 1 year. Once opened, refrigerate for up to 5 days.