Makes 1 quart
Using a damp cloth, remove dust and dirt from outsides of chiles. Stem and deseed them and cut or tear into rough pieces. Heat a cast-iron skillet over medium heat and add chiles; toast until fragrant, about 2 minutes. Place chiles in a bowl and add just enough hot water to cover. Cover bowl with plastic wrap and let chiles soften for at least 30 minutes but no longer than 1 hour. Place chiles in blender along with soaking liquid and puree to a very smooth paste. Set aside.
In a large, heavy saucepan, heat vegetable oil over medium heat. Add onion and cook for 3 minutes or until translucent. Add garlic, brown sugar, mustard, and sea salt and continue to cook, stirring, over medium heat for another 5 minutes. Add ketchup, 1 cup water, vinegar, molasses, tamarind paste, Worcestershire, cumin, cayenne, oregano, and chili paste. Bring to a boil over medium-high heat, reduce heat to medium low, and simmer, stirring occasionally, for 20 minutes. Taste and adjust seasoning if necessary. (The sauce can be stored in an airtight container in the refrigerator for up to 3 months.)