Wear a mask and gloves! The particles emitted from breaking the dried chile pods are spicy and could be very irritating to eyes and throat. Handling of the pods could cause your hands to burn depending on the heat of the chile.
Heat a large fry pan over medium heat. Dry roast the chile peppers in batches, until they release fragrance, about 1-2 minutes per side.
Once the chile peppers have been toasted and removed from pan, add cumin seeds. Shaking the pan, toast until fragrant. This should take about 1 minute.
Once the chiles are cool enough to handle, break the tops off and shake out the seeds.
Place chile peppers in a blender and pour in half of the chicken stock (or water) and toasted cumin seed. Puree until smooth but with a few specs of the pepper remaining.
Warm oil in a large saucepan. Cook the chopped onion until soft, about 5 minutes. Add garlic and cook until fragrant. Pour in the blended chile mixture.
Puree remaining chiles with remaining chicken broth and pour the puree into the sauce in pan. Add Mexican oregano and salt.
Reduce heat to medium-low and simmer for for 20 - 25 minutes. After about 15 minutes taste the sauce and adjust seasonings to your liking.
When sauce is ready it will be cooked down enough to coat a spoon thickly but still drop off easily.
Use the sauce warm or refrigerate for later use. The sauce keeps for 5 - 6 days and also freezes well.