Prep. Wash the tomatoes and roughly chop them. Add them to a large stockpot set over medium heat.
Cook. Bring the tomatoes to a boil, stirring often to prevent sticking. Once boiling, reduce the heat to medium-low and simmer uncovered for 30 minutes, stirring occasionally.
Mill. In batches, run the softened tomatoes through a food mill to remove skins and seeds. Discard solids or save for tomato powder.
Reduce. Return the smooth tomato pulp to the pot. Stir in lemon juice and salt (if using). Simmer over medium-low heat for 2½ to 4 hours, or until the sauce reaches a thick, paste-like consistency. It should mound up on a spoon.
Prep jars. While the sauce reduces, wash your jars and place them in the water bath canner. Cover with water and bring to a low simmer to keep the jars hot.
Fill. When the sauce is ready, remove jars from the canner. Ladle the hot tomato sauce into hot jars using a canning funnel, leaving ½ inch headspace.
Debubble. Use a debubbler or chopstick to release trapped air. Tomato paste is thick, so tap the jars gently if needed to help remove bubbles.
Seal. Wipe rims with a clean damp cloth. Place on lids and screw on rings to fingertip tight.
Process. Place jars into the boiling water bath, ensuring they’re covered by at least 1 inch of water. Process for 45 minutes for half-pints or pints.
Cool. After processing, turn off the heat, remove the lid, and let jars sit for 5 minutes. Then remove jars to a towel-lined counter and let cool undisturbed for 12–24 hours.
Store. Check for seals. Label, remove rings, and store in a cool, dark place for up to 18 months. Unsealed jars should be refrigerated and used within 3 days.