How to Make It
In a large heatproof bowl, cover the dried chiles and raisins with the boiling water; set a plate on top to submerge the chiles. Let stand until softened, about 15 minutes. Drain the chiles and raisins and transfer to a blender; reserve 1/4 cup of the soaking liquid.
In a small skillet, toast the sesame seeds and cumin over moderately high heat until fragrant and golden, shaking the pan, about 2 minutes. Transfer the seeds to the blender. Add the tomato, onion, garlic, lime juice, 1/4 cup of reserved soaking liquid and 1/2 cup of water and puree until smooth. Strain the salsa through a fine sieve, pressing hard on the solids. Season the salsa with salt.
Dried pasilla and guajillo chiles are available at Latin American markets, specialty food stores and some supermarkets.