To prepare tomato puree from frozen tomatoes: (assuming they were cleaned and cored before freezing) leave to thaw 24 hours; drain accumulated juices and put through a strainer to remove seeds and peels (like a Victorio hand-crank or a tripod metal food mill/press). You may need to heat the tomatoes briefly for easier straining, but only until just warmed.To prepare tomato puree from fresh tomatoes: wash, core and halve tomatoes; heat to boiling (I use a 12-qt. stockpot and a 6-qt. soup pot for 22 lbs); put through a strainer to remove seeds and peels.Make the pizza sauce: in a 12-qt. stockpot cook onions and garlic in the olive oil over medium heat until the onion softens. Add tomato puree and all the seasonings (basil through pepper flakes), bring to a boil, reduce heat and cook for about 30 minutes, uncovered. Process with an immersion blender to make a smoother sauce (or carefully blend in batches).Bring back to a boil; reduce heat and simmer, uncovered, until sauce reaches desired consistency (reduced by ¼ to â…“), usually an hour or two. Make sure to stir occasionally to prevent sticking (if you have a thin-bottomed stock-pot, stir more often to prevent scorching).To can: add the lemon juice or citric acid to each clean, warm jar, fill jar with sauce leaving ½-inch headspace and attach lids and rings until fingertip-tight. Place in canner rack and repeat with remaining jars to fill canner (keep sauce, lids and jars warm, as you will need to do a couple canner loads when using half-pints). Process both pints and half-pints for 35 minutes in a boiling-water canner (find more details in our Canning 101 steps found in the recipe index).Note: to prevent siphoning, recent recipes have you leave the jars in the water for 5 minutes after removing lid and turning off heat, though I don't always do this and I rarely have a problem with unsealed lids.Remove to a towel-lined surface and let sit undisturbed for 24 hours before checking seals, labeling and storing in a cool, dark place.