STERILIZING THE GLASS JARS:
Make sure you’re using glass jars and that they are proper canning because they will face high temperatures.
Firstly, wash the jars and lids well with soap and water, rinse them very well, then either place the jars in a tray and put them in the oven at 160-180ºC (320/356°F) for 15 minutes, or you can boil them for 15 minutes, or you can put them in dishwater in the highest temperature for about 40 minutes to 1 hour. Then remove them from the dishwasher, place them in a clean tea towel and allow them to dry completely. Don’t forget to sterilize the other utensils you’re going to use for canning the sauce.
MAKING THE TOMATO SAUCE:
First, roughly chop the tomatoes, don’t worry about the seeds and skin, we’ll deal with it later in a much faster fashion than peeling and seeding each tomato now. Chop all the other veggies too, except the garlic.
To a large pot over medium heat, add 1/4 cup of olive oil, when hot, add the diced onions, sprinkle with some salt, then sauté until golden. Add the carrots, celery, and garlic, sauté for about 5 minutes, then add the tomatoes, in layers along with salt on each layer, if your pot is too small, then add the bay leaf, and black pepper. Stir a bit, cover, and allow it to cook for 1 hour, stirring occasionally. The sauce is supposed to reduce by half.
After 1 hour, open the pot, stir and allow it to cook for 40 minutes more until it the sauce thickens a bit more.
Now, place the food mill on top of a large pot, then add a few ladles of the sauce to it, spin the handle until you see no more tomato sauce but only skins and seeds, remove the skins and seeds from the mill and discard it. Repeat the process until all the sauce has been processed.
Now, to thicken up the sauce we’ll cook it some more, for about 30 minutes more, over medium-low heat.
Now that the sauce has thickened up, transfer it to the jars, add some olive oil, (that’s optional and tastes very good) close the lids well, and now we’re going to provide a vacuum seal to the jars by boiling them this way they will last for months and months.
VACUUM SEALING THE JARS:
Take a large pot, place a tea towel in the bottom so the jars don’t crack while boiling, add the jars with the tomato sauce to the pot, cover the pot with hot water, turn on the heat and allow the jars to boil for 15 minutes.
Carefully remove the jars from the water, and place them upside down on a couple of tea towels, (the hot jars cannot be in touch with a cold surface otherwise the glass could crack due to the abrupt temperature change) now cover them with a towel or a few tea towels and allow the jars to cook completely, ideally 12 hours.
Now, you need to check if all the lids have vacuum sealed properly, just press the top, the lid is supposed to be firm and not bend, if they bend and you hear a clicking noise, this tomato sauce can be used in a few days but cannot be shelf stored.
Provided that your canned tomatoes were properly boiled, are kept in a clean, cool, dry place, and there’s no damage on the jar and lids such as dents, rust, swelling, or cracks, they can last indefinitely, however, I strongly recommend consuming them within 24 months. After opening, store it in the fridge and it will last for up to 5 days.
This tomato sauce is also freezable, it will freeze well for up to 6 months.