Start by heating a large skillet over medium heat. Add the ground beef and begin breaking it up with a wooden spoon or spatula. Cook the meat until it's fully browned and no pink remains, about 6–8 minutes. Once done, carefully drain any excess grease to prevent the dish from becoming too oily.

Add the finely chopped onion directly to the browned beef in the skillet. Stir and let it cook for 2–3 minutes until the onion softens. Next, stir in the dijon mustard, Worcestershire sauce, apple cider vinegar, and brown sugar. Let everything simmer together for 3–5 minutes, stirring often. This step lets all those savory and tangy flavors soak into the meat.

Now, give the mixture a taste and add salt and pepper as needed. This is your chance to adjust the balance—maybe a touch more vinegar if you like it tangy, or sugar if you want it sweeter. Stir well so that all flavors are evenly distributed throughout the mixture.

Place the bottom half of each slider bun on your serving tray or plate. Spoon a generous amount of the seasoned beef onto each bun. If you're adding cheddar cheese, sprinkle it on now so it can melt slightly into the warm meat. Top each one with a pickle slice if desired, then cover with the top half of the buns.

These sliders are best served immediately while warm and juicy. Pair with potato chips, coleslaw, or even a side salad for a complete meal. If you're prepping ahead, keep the meat warm on low heat and assemble just before serving for the best texture.