In a skillet over medium heat, cook the ground beef and diced onion until the beef is browned and the onion is softened. Drain any excess grease.

Season the beef mixture with garlic powder, salt, and pepper to taste. Remove from heat.

Slice the slider buns in half horizontally and place the bottom halves in the prepared baking dish.

Spread ketchup, mustard, and mayonnaise evenly over the bottom halves of the slider buns.

Spoon the cooked beef mixture onto the bottom halves of the slider buns.

Place a slice of cheddar cheese on top of the beef mixture.

Place the top halves of the slider buns over the cheese to form sandwiches.

Brush the melted butter over the tops of the slider buns.

Cover the baking dish with aluminum foil and bake in the preheated oven for 10-15 minutes, or until the cheese is melted and the sliders are heated through.

Remove the foil and bake for an additional 5 minutes, or until the tops are golden brown and crispy.