Line a gas grill with foil and turn up the edges just a bit. Line up the tomatillos, onion, and chillis on the foil, and turn the head of garlic top-down. Grill over medium high heat, turning everything except the garlic to get a good char on all sides. Remove veggies as they get the color you want. Turn off the heat and allow veggies to cool a bit. (If you don’t have a grill, line a baking sheet with foil and use your broiler to char the veggies, turning them until you get a good char on all sides).
Blend in batches with the salt and pour it all into a big bowl.
Heat the oil over medium heat. When the oil is hot, add the salsa all at once to shock it, then bring it to a simmer and cook for 20-30 minutes, adding in the chopped cilantro about halfway through the cook.
Pour into hot sterilized pint jars, top with lids, and process in a water bath canner for 20 minutes. Remove from the canner, adjust rings, and allow to cool. Will keep for one year.