Heat oven to 300˚F (149ºC) with rack set in middle position.
In a large, heavy-duty Dutch oven over medium heat, toast 1 ½ teaspoons cumin seeds and 1 teaspoon coriander seeds until fragrant and starting to lightly toast, 3–5 minutes. Set aside and allow to cool. Once cool, transfer spices to a mortar and pestle (or spice grinder) and grind until fine in texture.
Combine ground cumin and coriander with 1 teaspoon dried oregano, 1 teaspoon smoked paprika, 1 teaspoon kosher salt and ½ teaspoon cayenne pepper; set aside for later.
Trim 4-pound brisket of any excess fat or sinew (should end up with about 3½-pounds). Season generously with 2 teaspoons kosher salt and a generous amount of freshly cracked black pepper.
Heat 1 tablespoon oil in a large* heavy-duty Dutch oven over medium-high heat until shimmering. Add brisket and sear until deeply browned on both sides, about 5–7 minutes. Transfer to a plate.
Reduce heat to medium-low and add 1 sliced onion, stirring to deglaze the pan (add a splash or two of beef broth to help things along). Cook until onions are softened, about 5 minutes, then stir in 4 smashed cloves garlic and 2 tablespoons tomato paste. Cook 2 minutes, then stir in remaining beef broth. Cook until thickened and bubbling, 1–2 minutes, then stir in prepared spice mixture and 2 bay leaves. Nestle brisket (and any accumulated juices) into Dutch oven, spooning some onions and juices over top.
Cover Dutch oven and transfer to preheated oven. Cook for 3 hours, flipping brisket over halfway through, until brisket is tender and shreds easily with a fork.
Remove Dutch oven from oven and discard bay leaves. Using two forks, shred meat directly in Dutch oven. Return Dutch oven to oven, uncovered, and cook until liquid reduces slightly and meat is very tender, about 30 minutes. Cover and keep warm until ready to serve.
To serve, spoon shredded beef into a soft or hard tortilla and top with sliced avocado, shredded cheese, cabbage, and pickled jalapeños.