Wipe dried chiles with a damp paper towel to remove dust and dirt.
Heat a heavy-bottomed or cast-iron skillet over medium heat, let it heat up for a few minutes. Place the dried chile peppers, toast for 15-20 seconds on each side, pressing with a spatula. Transfer to a bowl and set aside to cool down. Do this in batches if required.
Or place the chiles on a baking sheet in a single layer and toast it in a preheated oven 350°F for 3-5 minutes.
Remove and discard the stems, shake out the seeds. Cut open the pepper with a pair of kitchen scissors and remove the membrane.
Transfer the chiles to a big heatproof bowl. Add boiling water. Cover and let it sit for 30 minutes or until they soften and turn plump.
Transfer the chiles into a blender, add the onions and garlic. Taste the soaking liquid, if it is bitter, do not use, add fresh water/stock. If it is not all that bitter, then add about 2.5 to 3 cups of soaking liquid and grind until smooth.
Place a fine-mesh strainer over a big bowl. Add the sauce in batches, extract out as much of the pulp by pushing it down with the back of a spoon or spatula. Use a clean spoon to scrape the pulp on the bottom of the strainer.
Heat oil in a heavy-bottomed skillet, add the strained sauce. Fry the sauce for 1-2 minutes, add Mexican oregano, cumin, and salt.
Note: Please be careful here, and do it gently, as there will be some splattering as soon as the sauce hits the skillet.
Mix well and simmer uncovered for 15-20 minutes, stirring occasionally. Check for seasoning, adjust with more salt if needed.
Is the sauce bitter at this point? Add 1 teaspoon of sugar or honey or a small piece of dark or semi-sweet chocolate.
Sauce still thin? Continue simmering for a few more minutes, until it thickens.
Sauce too thick? Adjust the consistency by thinning it out with few tablespoons of water at a time.