This recipe is based on weight in order to maintain a ratio that is safe for canning from Mother Earth News. If you do not have a scale, consider this excellent simple salsa recipe from the National Center for Home Food Preservation instead. A paste-type tomato makes the thickest salsa. Feel free to use a mix of sweet and hot peppers to find a blend that suits your taste for mild, medium or hot salsa.
Preheat the oven to 425 degrees F. Arrange the tomatoes, peppers, onion and garlic (if using) in a single layer on individual sheet pans.
Roast the vegetables in batches until they are blistered all over, about 30 minutes. Weigh the vegetables once they are roasted to be sure you have the correct amount of each for the proper ratio.
Transfer the vegetables into a food processor in batches. Pulse to chop them until the texture is consistent but still chunky. Transfer each batch into a heavy-bottomed stock pot and repeat with the remaining vegetables.
Heat the salsa over medium heat, stirring often to prevent scorching on the bottom. Add the lemon juice and salt to taste. You may increase (but not reduce) the lemon juice and salt to suit your taste.
While the salsa is heating, bring the hot water canner to a boil. Ladle the salsa into clean and sterilized pint jars, leaving 1/2-inch head space. Wipe the rims, top with hot lids and screw on the rings finger tight.
Place the jars into the rack and slowly lower them into the hot water canner. Start timing when the water reaches a rolling boil, start timing and process the pints for 15 minutes (20 minutes from 1,000-6,000 feet; 25 minutes above 6,000 feet).
When the processing time is reached, lift the jars from the boiling water with the rack and transfer each jar to the counter lined with the dish towel. Do not disturb the jars for 24 hours. When cooled and sealed, remove the rings, label and store.