1. Bring 1 cup water to a boil in a small saucepan and add chiles, onion, and garlic. Cover, then reduce heat and simmer until chiles are very tender when pierced, about 15 minutes.
2. Whirl chile mixture in a food processor with vinegar and salt until fairly smooth.
Nutritional analysis is per Tbsp.
Make ahead: Up to 1 week, chilled.