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In a medium pot, add the chiles and onion then cover with filtered water.
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Bring to a boil then remove from heat. Let sit 10 more minutes.
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Remove the seeds and stems from the peppers then put peppers, onion, garlic, vinegar and salt in a food processor or blender. Puree.
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Strain through a fine-mesh colander over a bowl and press it through with a rubber spatula.
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Scrape the outside of the colander to get all the sauce you can. Stir in the water.
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Pour into a jar and refrigerate. It will thicken up in the refrigerator so you may want to thin it out some more.