Toast the dried chiles in a skillet over medium heat until fragrant, then soak them in hot water for 15 minutes.
Blend the soaked chiles with chipotle, adobo sauce, olive oil, garlic, vinegar, cumin, oregano, salt, and pepper until smooth.
In a large pot or Dutch oven, sear the beef chunks until browned on all sides.
Add chopped onion, chile sauce, beef broth, and bay leaf. Bring to a boil, then reduce heat and simmer covered for 3 hours or until meat is tender.
Shred the beef and mix it back into the broth.
Heat a skillet over medium. Dip tortillas in the broth, then place on the skillet.
Top each tortilla with shredded beef and Oaxaca cheese, then fold into tacos and cook until crispy.
Serve with extra broth for dipping and garnish with fresh cilantro and diced onions.