Start by filling the large water bath canner with water. Bring water to a boil over high heat.
While water is heating, peel the onions and cut into large chunks and place in a food processor. Pulse until the onions are very small. Pour chopped onions into a large stock pot.
Cut all peppers and remove seeds, then also place in a food processor and pulse until small. Transfer them to the large stock pot with onions.
Peel the garlic cloves and add them to the food processor. Pulse until garlic cloves are pureed. Add to stock pot with peppers and onions.
Add in apple cider vinegar, salt and sugar to the large stock pot.
When the water begins to boil, it's time to blanche the tomatoes. Submerge a few tomatoes in the boiling water (about 5-6 at a time). When the skin of the tomato splits, immediately remove the tomatoes from the boiling water and place them in the cold water. Repeat this process until all the tomatoes have been blanched and start to split.
Once the tomatoes are blanched, take a knife and cut the skins and the center core from the tomatoes. Place the peeled tomatoes in a food processor and puree or pulse until there are no longer any large chunks of tomatoes.
Pour the pureed tomatoes into the large stock pot with peppers and onions. Mix until well blended.
Bring the salsa to a boil, then turn heat to medium. Cook for 1 hour, stirring frequently to avoid burning on the bottom.
After 1 hour, add the tomato paste to the salsa mixture. Continue cooking for 1-2 hours until salsa has reached desired consistency. The salsa will continue to thicken the longer it's cooked.
Wash and prepare quart jars.
When salsa has a nice, thick consistency, remove from heat and fill jars immediately.
Using canning tools, fill each jar with salsa leaving 1/4" from the top.
Once the jars have been filled, wipe the top of the jar with a paper towel to remove any salsa that may have gotten on the rim of the jar.
Place the lids on and hand-tighten the rings.
Let the jars cool for 30 minutes, then place in them in the water bath canner.
Once the jars have been placed in the water bath canner, fill the canner with water until the jars are completely covered with at least 1" of water. (I like to place my finger on the top of the lid and if the water comes to my first knuckle, then it's enough water.)
Cover and bring the water to a boil and boil for 25 minutes.
When the time is up, using jar tongs (canning tools) to lift each jar out of the boiling water and set aside. Do not touch or move the jars after they have been removed from the water bath for 24 hours.
Once the jars have cooled completely, check to see if the jars have sealed properly by lightly pressing down in the center. If the lid clicks or pops up and down - it did not seal.
If it did not seal properly, store the salsa in the fridge.
Store jars of salsa in a cool, dry place.
Shelf stable for up to 2 years.