Season the roast generously. Pat the beef dry. Mix salt, pepper, smoked paprika, garlic powder, and onion powder.
Rub all over the roast, pressing it in to create a flavorful crust.
Sear for flavor. Heat olive oil in a heavy skillet over medium-high. Sear the roast 3–4 minutes per side until well browned. This step adds restaurant-level depth.
Build your base. Add sliced onions and minced garlic to the slow cooker.
Place the seared roast on top.
Make the sauce. In a bowl, whisk beef broth, brown sugar, Worcestershire, apple cider vinegar, soy sauce, mustard powder (or Dijon), and red pepper flakes. Pour around the roast.
Slow cook to tender. Cover and cook on Low for 8–10 hours or High for 4–6 hours, until the roast is easily shreddable with a fork.
Add the butter finish. In the last 30 minutes, add the butter on top of the meat. It melts into the sauce, giving a silky, steakhouse-style finish.
Shred and thicken (optional). Transfer the roast to a board and shred.
Skim excess fat from the cooking liquid. If you want a thicker gravy, whisk cornstarch with 2 tablespoons cold water and stir into the liquid. Cook on High for 10–15 minutes until glossy.
Serve. Return the shredded beef to the slow cooker and toss with the sauce.
Garnish with chopped parsley. Serve over mashed potatoes, buttered noodles, or Texas toast.