Rinse the chiles in cold water and pat dry.
Tear open the chilies, and remove the seeds and bulky veins (like the tops/stems)
Heat a large skillet over medium heat, and then toast the peppers, onions and garlic...turning chilies frequently..do this for about 10 minutes
Bring 3 cups of water to a boil...add the tomatoes for 30 seconds.
Remove the tomatoes into an ice bath...remove from the ice bath, and peel off the skin. Slice and half, and remove the ribs and seeds of the tomatoes.
Bring 4 cups of water, the 2 cups of beef stock and the tomato sauce to a boil
Transfer the chilies, onions, garlic and tomatoes to the water/stock. Add in the oregano and cumin. Lower the heat to low/medium and simmer for about 15 minutes
Carefully transfer the chili mixture to a blender, or larger food processor. Puree for about 2 minutes.
Add the mixture back to the pot through a very fine sieve, or a colander lined with cheese cloth. Press until all liquid has been pressed through. This is an important step! Don't skip this!
Return the strained sauce to the stove and heat for about 15 minutes. Add about a teaspoon of salt (or to taste)