Using a meat tenderizer (or fork), lightly prick the brisket all over.
Combine the chili powder, brown sugar, oregano, garlic powder, salt, cumin, black pepper, and cayenne pepper in a bowl.
Rub the mixture over the meat. Wrap tightly in plastic wrap and refrigerate overnight.
Arrange the onion in the bottom of the slow cooker.
Add the seasoned meat and top with the crushed tomatoes, beef broth, soy sauce, and white vinegar. Sprinkle the red pepper and jalapeño pepper on top.
Cover and cook for 10 hours on low, or 6 hours on high.
Remove the meat from the slow cooker and tent with foil to rest.
Strain the remaining liquid and discard the vegetables. Transfer to a saucepan and bring to a boil. Cook until reduced by half, about 10 to 15 minutes.
Heat the tortillas in a skillet. Shred the beef with forks.
Top the tortillas with the shredded brisket and drizzle with about a tablespoon of the reduced sauce. Top with cheese, onions, jalapeno, and avocado.