Ensure that you have a very clean work space including your counters, sink, and utensils for preserving the passata. Place the tomatoes in a sink full of water. Add a little produce wash if you like. Use a veggie scrub cloth to wash each tomato. Rinse well and place in a large bowl.
Assemble the following items on the kitchen counter: a large pot to hold the chopped tomatoes, a smaller bowl for everything you discard, a cutting board, a sharp paring knife and of course, your cleaned tomatoes. Use a paring knife, remove from each tomato the stem scar and all blemishes. Quarter the tomato lengthwise so it is easy to remove any fibrous part. If the tomato is large, you can quarter it. It is really important to not use tomatoes that show signs of decay or mold.
Once your pot is full of tomatoes, place the lid on it and set it on the stove on low-medium heat. Bring the tomatoes to a boil, stirring often to prevent them from scorching. As the tomatoes soften, they will take less space so you can add any remaining chopped tomatoes. Simmer for about 15 to 20 minutes. The tomatoes will slowly begin to exude water. Use a ladle to remove as much water as you can. Don’t be surprised if you remove several cups of liquid. You can use this tomatoey-water to flavour soups or cook rice at a later time or discard.
Remove the pot from the heat. Use an immersion blender to break up the tomatoes but do not purée it completely. Be careful to not burn yourself as the tomatoes may splatter.
Assemble the food mill using the fine sieve bottom plate. Place the assembled food mill on top of another large bowl or pot to fit snuggly. Using a large measuring cup or ladle, pour the cooked tomatoes into the food mill and start cranking the handle. Take precautions as the tomatoes can spit up a bit. Repeat this step until the accumulating seeds and skins are quite dry. Use a spatula to push the tomatoes that creep up the sides of the food mill on to the sieve plate to be processed by the food mill. Once the seeds and skins are dry, carefully tilt the food mill and with a spatula, scrape off the purée that has clung to the bottom of the sieve, allowing it to fall into the pot or bowl. Take care as to not let the remaining skins and seeds fall back in the purée.