1Remove stems and seeds from the guajillo chiles and ancho chiles. Place the chiles in a heatproof bowl, cover with hot water, and let them rehydrate for 20-30 minutes until softened. Drain the chiles, reserving a small amount of the soaking liquid.
2In a blender, combine the rehydrated guajillo chiles, ancho chiles, garlic, white onion, Mexican oregano, ground cumin, lime juice, vegetable oil, and salt. Add a splash of the reserved chile soaking liquid if necessary to achieve a smooth, thick paste. Blend until completely smooth.