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1
Adjust oven rack to lower middle position and preheat oven to 275u00b0F. Add dried chilies to large heavy-bottomed Dutch oven or stock pot and cook over medium-high heat, stirring frequently, until slightly darkened with intense, roasted aroma, 2 to 5 minutes. Do not allow to smoke. Remove chilies to small bowl and set aside. Alternatively, place dried chilies on a microwave-safe plate and microwave on high power in 15-second increments until pliable and toasted-smelling, about 30 seconds total. Transfer to a 2-quart microwave-safe liquid measuring cup or bowl. Add 2 cups chicken broth, cover with plastic wrap, and microwave on high power until gently simmering, about 5 minutes. Remove from microwave and set aside.
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2
Meanwhile, heat 1 tablespoon oil in now-empty Dutch oven over high heat until shimmering. Cook oxtails until well-browned on all sides, about 8 minutes total. Remove oxtails and set aside. Reduce heat to medium.
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3
Add remaining two tablespoons oil and heat along with onions and garlic and cook, stirring frequently, until deep brown and just starting to burn, about 10 minutes. Add cumin, cloves, and oregano and cook, stirring constantly until fragrant, about 30 seconds. Add chipotle chilies, vinegar, and remaining chicken broth. Scrape up browned bits from bottom of pan, simmer until reduced by about half, then transfer entire contents to the jar of a blender.
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4
Add soaked chilies and their liquid to the blender along with fish sauce. Start blender on low and slowly increase speed to high. Puree until smooth, about 1 minute. Set aside.
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5
Place beef chuck in Dutch oven. Add oxtails, bay leaves, and sauce. Bring to a boil over high heat. Place lid on pot slightly cracked, then transfer to oven. Cook, turning beef occasionally, until completely tender and a cake tester or metal skewer inserted into meat shows little to no resistance, about 4 hours. Discard bay leaves and oxtails (meat from oxtails can be eaten if desired). Transfer chuck to a large plate. Return Dutch oven to stovetop and cook, stirring frequently, over medium-high heat until liquid is reduced to about 1 1/2 cups, about 5 minutes.
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6
Beef can be cut and served immediately, but for best flavor, transfer beef to a sealed container along with liquid and refrigerate up to five days. When ready to serve, slice beef against the grain into 1 1/2- to 2-inch slices, then shred into large chunks with fingers or two forks. Return beef to a pot along with the sauce. Bring to a simmer and cook, gently stirring and folding until beef is hot, tender, and coated in sauce. Season to taste with salt. Serve immediately, piling the beef into warm corn tortillas with onions, cilantro, salsa, limes, or other condiments as desired. Alternatively, serve as-is with rice, noodles, grits, or cornbread.
