Dice the tomatoes and onion on a cutting board and set them in a large bowl. Keep as much of the juice from the tomatoes you can.
Pour the juice off the tomatoes in the bowl straight into the food processor. Add your garlic cloves, whole serrano peppers and cilantro. Give it a whizz for 15-20 seconds if not more.
Combine the contents of the food processor with the tomatoes - in the same bowl. Mix it up with a spoon or clean hands.
Weigh out the contents of the bowl and add 2% of the weight in salt. Or calculate 2 Tbsp of salt for each quart of salsa.
Sprinkle the salt on the contents of the bowl. Work it in with clean hands. Lightly cover the mixture and let it sit for 20-30 minutes.
Pack the mixture between 2 wide mouth quart jars and one pint jar. Place a jelly jar in the mouth of each jar, taking care to push down the contents of the jar so that the salsa is under the natural brine (no additional water should have to be added because there should be enough brine).
Place an airlock on each jar. Allow to sit for 3-4 days - over time you'll see bubbling activity. Remove the airlock and place a regular canning lid on each jar.
Refrigerate and consume within 5-6 weeks. Refrigeration will slow down, but not halt, the fermentation process.