First, prepare your sauce: Remove the stems and seeds from the dried chiles and place into a large sauce pan. Add the onion and garlic. Cover with water. Bring to a boil, reduce to a simmer and continue cooking for 10 minutes. Stir halfway through cooking time. Remove from heat and let cool in pan for 20 minutes.
While you wait, slice the brisket into 6 equal chunks. Season lightly with salt, pepper, garlic and chile ancho. Set aside.
Using a slotted spoon or tongs, transfer the chiles, onion and garlic to the blender. Add 3 cups of water or beef broth to the blender. Add oregano, cumin, pepper and salt. Blend on high for a minute straight. Scrape down the sides of blender and blend on high for another minute. Strain, using a wire mesh strainer, into a bowl. Set aside.
In a large pot, preheat 4 tablespoons of olive oil to medium heat for 5 minutes. Sear the beef for 3-4 minutes per side. Remove from pot onto a plate.
In that same pot at medium heat, add the chile sauce from the blender. Stir to scrape off the bits on the bottom of the pot. Add 3 more cups of water or broth. Bring to a boil. Add the beef back in, making sure it is immersed in the sauce. Cover and cook and a steady simmer for 5 -6 hours or until the brisket if pull apart tender.