Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the ground meat, diced onions, ketchup, mustard, salt, and pepper. Mix until well incorporated.
Heat olive oil in a skillet over medium heat. Add the meat mixture and cook until browned, approximately 6-8 minutes. Use a spatula to break up the meat as it cooks.
Once cooked, remove the skillet from heat and allow the mixture to cool slightly.
Slice the slider buns in half horizontally and place the bottom half on the prepared baking sheet.
Spoon an even amount of the meat mixture onto each of the bottom bun halves. Then, top with shredded cheddar cheese.
Place the top half of the buns over the cheese to create sliders.
Bake in the preheated oven for about 10-12 minutes or until the cheese is melted and the buns are lightly toasted.
Remove from the oven and let cool for a few minutes before serving. Optionally, garnish with pickles.