Soak the Guajillo chili peppers in water for a couple of hours. The seeds should float to the top. Discard the stems and pods and clean off any remaining dirt. Wash the cilantro leaves in a colander and set aside.
In 3 tablespoons of oil, saute the onions till soft and translucent, on a medium heat, stirring often. Turn the heat down a bit and add the cilantro and chili peppers. Keep stirring and saute for 10 minutes, then add the Ro-Tel, cumin & cayenne, and let simmer for about 20-25 minutes more, covered, on a low heat, stirring occasionally. Once the flavors go through, give it a final stir and transfer the whole thing to a blender. Add 3 cups of chicken stock and blend on low till it forms a paste.