01 - Preheat oven to 300°F. In a small bowl, combine kosher salt, black pepper, smoked paprika, garlic powder, onion powder, and ground cumin. Mix thoroughly to create a uniform dry rub.
02 - Rub the spice mixture evenly over all surfaces of the trimmed beef brisket, pressing gently to adhere. Heat olive oil in a large Dutch oven or oven-safe pot over medium-high heat. Sear the brisket on all sides until deeply browned, approximately 2-3 minutes per side.
03 - Transfer the seared brisket to a plate temporarily. Add sliced onions and smashed garlic to the rendered fat in the pot; sauté for 2 minutes until fragrant and slightly softened. Return the brisket to the pot and pour in the beef broth.
04 - Cover the pot tightly with a heavy lid. Transfer to the preheated oven and cook for 4 hours, turning the brisket halfway through the cooking time. The meat is done when it yields easily to a fork and can be pulled apart.
05 - While the brisket cooks, combine ketchup, apple cider vinegar, brown sugar, Worcestershire sauce, Dijon mustard, smoked paprika, garlic powder, onion powder, and cayenne pepper in a medium saucepan. Simmer over low heat for 15 minutes, stirring occasionally, until the sauce thickens and coats the back of a spoon. Season with salt and pepper to taste.
06 - Remove the brisket from the oven and let it rest for 15 minutes. Using two forks, shred the meat, discarding any large pockets of excess fat. Toss the shredded brisket with half of the prepared BBQ sauce until evenly coated.
07 - Brush the cut sides of the slider buns with melted butter. Arrange on a baking sheet and toast lightly under the broiler or in the oven until golden brown, about 2-3 minutes. Watch closely to prevent burning.
08 - Place a generous portion of sauced brisket on the bottom half of each bun. Top with a halved slice of cheddar cheese followed by a spoonful of coleslaw. Drizzle with additional BBQ sauce as desired. Cover with the bun tops and serve immediately while warm.