One bite from these shredded beef tacos with horseradish sauce and you'll fall in love with this succulent British and Tex-Mex fusion.
Cut the onions
Cut a medium to large onion into 1/4-inch slices and discard the onion ends. Stack the onion slices and cut them in half vertically, creating two half onion stacks. Cut each of those stacks in half and place at the bottom of the slow cooker.
Brown the beef
Trim the roast using a sharp knife and discard any excess fat. In a small bowl, combine the salt, pepper and paprika. Rub the entire chuck roast with 1 tablespoon of vegetable oil and coat with the salt mixture. In a heavy-bottomed pan over medium-high heat, add the remaining oil and brown the roast on all sides. Resist the temptation to turn the meat prematurely; a proper sear is needed to develop full flavor. When the meat is completely seared on all sides, reduce the heat to medium and transfer the roast to the slow cooker (place on top of the onions). You may notice that the roast will have a tendency to stick to the pan. This is normal. Let the meat continue to cook and when the meat is properly seared it will naturally release from the pan. Keep turning the meat until all of the sides have released and the roast is completely browned.
Slow cook the beef
In the same heavy-bottomed pan used to sear the roast (now over medium heat), add the 2 tablespoons of apple cider vinegar to the pan and mix together the vinegar and oil with a spatula, scraping up and incorporating as much of the browned glaze and bits (or "fond") as possible. After 2 to 3 minutes, pour the deglazed vinegar and oil into the slow cooker and over the roast. In the cup of beef broth, whisk in the chili powder and cumin until well combined and pour into the slow cooker. Add the garlic and bay leaves, cover the slow cooker and set to the low temperature setting. Cook for 6 to 8 hours or until the beef can be easily shredded with a fork.
Shred the beef using two forks, discarding the garlic cloves and bay leaves as well as any gristle or fat you may find. Optional (but awesome): In the last hour of cooking, remove and shred the beef and return the beef to the slow cooker and let it cook for the remaining time without the lid. The beef will soak up more of the rich broth and the extra sauce will thicken and become much more flavorful.
In a small bowl, combine the sour cream, prepared horseradish, lemon juice, salt and Worcestershire sauce until the ingredients are completely incorporated. Cover and refrigerate until you are ready to serve (refrigerate the sauce for at least 30 minutes for best results).
Assemble the tacos
In batches of four, warm the tortillas in the microwave by covering them with a damp paper towel and cooking 30 seconds at a time until the tortillas are completely warmed through. On each tortilla, place about 3 to 4 ounces of shredded beef with a small spoonful of the beef juices from the slow cooker. Add the shredded cheese, and sour cream sauce. Garnish with the green onions.