How to Make It
In a large heatproof measuring cup, soak the chipotles in 1 cup of very hot water until softened, about 20 minutes. Remove the chipotles from the water and discard the stems; transfer the chipotles to a small bowl and toss with the oil. Reserve half of the chipotle soaking liquid (about 1/2 cup).
Preheat the broiler. On a large rimmed baking sheet, arrange the tomatoes, onion and garlic. Broil 6 inches from the heat until the vegetables are nicely charred, about 12 minutes. During the last 2 minutes of broiling, add the chipotles to the baking sheet.
Scrape the roasted vegetables and any accumulated pan juices into a blender. Add the reserved soaking liquid, tamarind purée and salt, and purée until smooth.