Wash the tomatillos with cool water and pat dry. Remove stem from chiles, and deseed for mild salsa. Peel the garlic cloves.
Heat a cast iron skillet on high until very hot. Add the tomatillos to the skillet and roast them until blistered with black charred spots. Use tongs to carefully turn them occasionally.
Turn heat to medium and add chiles, garlic cloves and onion. Roast until they also are blistered with charred spots and have softened; about 5 minutes.
Take skillet off heat and add water to loosen any charred pieces stuck to the bottom.
Let everything cool a little, then add contents of skillet (tomatillos, chiles, garlic, and onion, including water), cilantro and sea salt to a blender. Blend for about 30 seconds until fairly smooth.
Heat oil in the skillet, then pour in the blended salsa mix. Bring to a boil, then simmer until salsa thickens; about 5 minutes.
Use right away, or cool and store refrigerated in an airtight container.
