Place the dried chili de arbol, guajillo, and pasilla chilies in a heat safe bowl and add enough boiling water to cover the chilies. Place another bowl on top to keep the chilies submerged. Let sit for 30 minutes. Drain the water and discard the stems and seeds. Set aside.
Season the brisket generously with salt and pepper. Heat 2 tablespoons of oil in a Dutch oven over medium-high heat. Add the brisket and sear on all sides until seared. Remove from heat and set aside.
Reduce heat to medium. In the same pan, add the chopped onion and saute until softened, about 4-5 minutes. Add the garlic, coriander, cumin, oregano, and tomato paste, stirring to combine. Add back the brisket along with the rehydrated chilies, cinnamon stick, bay leaves, and chicken stock. Cover and bring to a boil. Turn off the heat and continue cooking the brisket in the oven for 2 ½ hours or until the meat is fall-apart tender.
Remove the brisket from the pot and shred the meat using two forks. Strain the braising liquid, discarding the ingredients but keeping the liquid. Ladle ½ cup of the braising liquid over the shredded beef, tossing to coat.
Combine chopped onion and cilantro in a medium bowl. Season with lime juice and salt. Set aside.
Heat a nonstick saute pan over medium heat. Carefully dip the tortilla in the top part of the braising liquid where the fat resides. Place the tortilla in the pan and cook for 1 minute or until lightly browned. Flip over and top with shredded brisket and shredded cheese on one half of the tortilla. Fold over to create a taco and cook until lightly browned on both sides. Remove taco and repeat with remaining ingredients.
Serve with warm au jus for dipping and onion cilantro topping.
Notes
You can braise the brisket ahead of time and assemble the tacos right before serving.