Preheat oven to 350°F.
Add the beef chuck roast to an oven-safe dish with a lid.
Season both sides of the beef with salt, pepper, ancho chili powder, and chili lime seasoning.
Place the garlic heads and shallots in the dish, and optionally pour in ⅓ to ½ cup of beef broth.
Cover and bake for 2.5 hours.
Make the sauce by blending together the cilantro, jalapeño, sour cream, white vinegar, lime juice, salt, and pepper.
After 2.5 hours, remove the beef from the oven, shred it using two forks, and mix it with the garlic and drippings.
Return the shredded beef to the oven at 450°F for 12-15 minutes.
Serve the beef with the cilantro sauce, avocado, and chili flakes.
