Shredded Beef Taco Bar

Preheat the oven to 325°F (165°C).
In a mortar, pound the chiles until finely ground. (Alternatively, you can blitz them in a coffee or spice grinder.)
In a large enameled cast-iron Dutch oven set over medium-high heat, warm the oil. Season the shank on all sides with salt and pepper. Add the shank to the Dutch oven and cook until browned on all sides, about 15 minutes. Transfer the shank to a plate.
Add the onion, garlic, and a pinch of salt to the Dutch oven, reduce the heat to medium, and cook, stirring occasionally, until the onion begins to brown, 5 to 6 minutes. Stir in the chiles, oregano, and cumin and cook, stirring constantly, until fragrant, about 1 minute. Add the beer and bring to a boil, scraping up any browned bits with a wooden spoon. Cook until the beer is reduced by half, 6 to 8 minutes. Pour in the crushed tomatoes and chicken stock and return to a boil.
Return the shank and any accumulated juices to the Dutch oven. Cover and braise in the oven, basting with the sauce in the pan every hour, until the meat is very tender, 3 to 3 1/2 hours. Remove from the oven and let the beef rest at room temperature, in the braising liquid, uncovered, for at least 2 hours. (The beef can be braised and stored in the refrigerator, in its braising liquid, for up to 4 days. Before proceeding, skim any congealed fat on the surface, cover, and rewarm gently over medium-low heat.)

In a medium bowl, whisk the vinegar, sugar, and salt until the sugar dissolves. Add the sliced onion, cover, and let stand at room temperature, stirring occasionally, for at least 1 hour. (The pickled red onion can be refrigerated for up to 2 weeks.)

In a small bowl, whisk together the sour cream, zest, and juice and season with salt and pepper. Refrigerate until ready to serve.

In a large bowl, whisk together the mayonnaise with hot sauce and season with salt and pepper. Add the red cabbage and toss to coat. Refrigerate until ready to serve.

Move the shank from the braising liquid to a plate. Pour the braising liquid into a medium saucepan and bring to a simmer over low heat, skimming any accumulated fat. Stir in the vinegar and butter and season with salt and pepper. Keep warm.
If you’d like crispy, slightly charred beef, light a grill. Fold a 24-inch (60-cm) sheet of foil in half and brush lightly with oil. Place the foil, oiled side up, on the grill. Place the cooled shank on the foil, close the lid on the grill, and cook over moderately high heat, basting with warm braising liquid every 10 minutes, until the shank is lightly charred and warmed through, 35 to 40 minutes. Move to a platter and let rest for 10 minutes. Discard the strings and use a couple forks to shred the beef and pile it on a platter. If you’d prefer softer shredded beef, discard the strings from the beef and use a couple forks to shred the meat. Stir the shredded beef into the warm braising liquid and gently stir until warmed through. Using a slotted spoon, pile it on a platter.
Serve the shredded beef warm with pickled onion, crema, slaw, flour tortillas, cilantro, lime wedges, and remaining braising liquid for each person to make their own tacos.