How to Make It
Heat a large skillet over medium. Working in batches, add chiles to skillet, and cook, turning often, until slightly puffed, toasted, and fragrant, 1 to 2 minutes.
Return all chiles to skillet, and add water just to cover. Bring to a simmer over medium-high. Add onion and garlic; reduce heat to medium-low, and simmer until onion is tender and chiles are soft and pliable, 10 to 15 minutes. Remove from heat, and let cool slightly, about 15 minutes. Reserve 1/2 cup cooking liquid.
Using a slotted spoon, transfer vegetables to a blender. Add reserved 1/2 cup cooking liquid, and process until mixture is a coarse paste, about 8 seconds. Add vinegar and salt, and process until mixture (except for seeds) is smooth, about 15 seconds. Let cool before serving.