1. Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
2. Prepare the beef patties: In a mixing bowl, season the ground beef with salt, pepper, and finely chopped onions. Mix gently and form into 12 small, evenly sized patties.
3. Cook the patties: Heat a skillet over medium-high heat and cook the patties for about 3-4 minutes per side until browned and cooked through.
4. Assemble the sliders: Slice the slider buns in half. Place the bottom halves on a baking sheet.
5. Layer the beef and cheese: Place a cooked beef patty on each bun bottom, top with a slice of cheddar cheese, pickles, and condiments like ketchup or mustard.
6. Top with the bun lids: Place the top halves of the buns on each slider.
7. Bake: Brush the tops with melted butter and bake in the preheated oven for 8-10 minutes, until the cheese melts and the buns are toasted.
8. Serve hot: Remove from the oven and serve immediately for a cheesy, savory experience.

For additional flavor, consider adding your favorite toppings like crispy bacon or fresh lettuce. Using quality ingredients like high-quality ground beef and fresh cheese slices will elevate your burger bites.