2While chiles rehydrate, roughly chop the Roma tomatoes, White onion, and Garlic cloves.
3Drain the rehydrated Guajillo chiles and Ancho chiles, reserving a small amount of the soaking liquid. Transfer the chiles, Roma tomatoes, White onion, and Garlic cloves to a blender. Add 100g of the reserved chile soaking liquid and blend until very smooth. Strain the mixture through a fine-mesh sieve into a bowl, pressing on the solids to extract all liquid, and discard the solids.