Roast the onion and garlic in a little olive oil in a 450 degree F oven for 15 minutes.
Preheat the oven to 300 degrees F. Sear the beef in bacon fat or vegetable or olive oil on both sides in a Dutch oven.
In a clean skillet heat the peppers over medium heat until they are fragrant. Cool, then remove the seeds and stems. Blend the spices, chilis, roasted onion and garlic, and 1 cup broth. Pour the blended sauce over the top of the roast. Add the additional water or broth and 2 Tbsp vinegar to the Dutch oven.
Cover and roast at 300 degrees for 3-4 hours, spooning the sauce over the meat several times, until tender. If using a cast iron or oven safe skillet, cover with foil. The meat will be super tender when pulled apart with a fork. Salt and pepper to taste.
Proceed to make the tacos: If desired remove a bit of the fat from the Dutch oven and discard. Remove the beef to a plate. Shred the beef using two forks or a gloved hand. Heat a skillet with a spray of oil. Add 2 or 3 corn tortillas. Turn over a few times. Add a small amount, a teaspoon of the birria sauce, then add some of the beef and a small amount of cheese to the inside, then fold over. Add cheese to the outside and cook until browned. If desired, add a small amount of sauce to the outside or serve the sauce alongside the tacos, for dipping.
Serve with finely shredded cabbage, tomato, avocado, and salsa.