Heat a large skillet over medium-high heat. Add the ground beef and diced onion.
Cook until the beef is fully browned and the onions are tender, breaking the meat into very small pieces as it cooks. Drain any excess grease if necessary.
Reduce the heat to medium. Add the beef broth, Worcestershire sauce, yellow mustard, brown sugar, apple cider vinegar, salt, and black pepper.
Stir everything together and simmer gently for 10–15 minutes, or until most of the liquid has been absorbed by the beef.
Turn off the heat and allow the mixture to rest briefly.
Spoon a generous portion of the warm loose-meat mixture onto the bottom half of each slider bun.
Add dill pickle slices and a little extra yellow mustard, if desired. Place the top buns over the filling and serve warm.
