Remove the peel from the onion and cut off the ends, then cut in half lengthwise
Slice the serrano and jalapeno in half / I keep the seeds & rib in place. If you do not want it too spicy then please remove the rib and seed
The Guajillo peppers, remove the stem and seeds
Heat cast-iron skillet to medium-high heat. Once hot, add the onion face down, peppers face down along with the dried Guajillo peppers
Pan roast until charred. You want to see charr on both sides of the onion. The onion needs to be charred to reach full flavor. Keep an eye on the dried Guajillo so they do not burn. Once you begin to smell the Guajillo aroma in the air the Guajillo only needs a short amount of time until correctly toasted. The serrano and jalapeno should take on some charr before removing from the heat too.
Add all charred ingredients to a blender and add the canned tomatoes and Rotel along with a pinch of salt. Blend until the consistency you enjoy. You can always chop up some fresh tomatoes and onion for more texture.
That's it. The flavor will develop overnight