Caramelize the Onions: In a skillet over medium heat, add the olive oil. Once hot, add the sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions are soft and golden brown, about 15-20 minutes. Add the balsamic vinegar and sugar, stirring until the liquid has evaporated and the onions are caramelized. Remove from heat and set aside.
Prepare the Beef Patties: While the onions are caramelizing, form the ground beef into 4 equal patties. Season both sides with salt and pepper. Press a small indentation in the center of each patty to prevent them from puffing up during cooking.
Cook the Sliders: In a clean skillet or grill over medium-high heat, cook the patties for about 3-4 minutes on one side. Flip them over, place a slice of cheddar cheese on each patty, and cook for an additional 3-4 minutes until the cheese is melted and the beef is cooked to your desired doneness.
Assemble the Sliders: Cut the slider buns in half. On each bottom bun, place a beef patty topped with caramelized onions. Add a handful of fresh arugula or spinach for crunch.
Add Sauces: If desired, drizzle some BBQ sauce or spread a thin layer of mustard on the top bun before assembling.
Finish: Place the top bun on the sliders and serve immediately while hot.