In a large stock pot over medium heat, bring the tomato sauce to a simmer.
Add the diced onions, diced jalapeños, minced garlic, and chopped cilantro.
Allow to simmer for 30 minutes while preparing the water bath and canning jars.
Fill the water bath and bring to a boil over high heat.
Wash the jars, lids, and rings in hot soapy water.
In each clean jar, add 1/2 tsp of canning salt and 1 tbsp of lemon juice.
Using the food funnel, fill each jar with salsa, leaving 1/2 inch head space at the top. Run the bubble popper through the jar to release any trapped air.
Wipe the rims of the jars with a clean, dry paper towel and place the lids on the jars. Tighten the lids with the bands only fingertip tight.
Using the jar lifter, place the jars into the boiling water bath, cover and process for 35 minutes for sea level altitude. (adjust for higher altitude)
Once the jars have been processed, remove the water bath from the heat and carefully remove the jars to a towel lined cutting board to cool.
Allow the jars to rest for at least 12 hours before labeling, dating, and storing. Any jars that did not seal should be refrigerated and used within 10 days.