Pull-Apart Cheeseburger Sliders

Preheat oven to 350° and grease a 9"-x-13" baking dish with cooking spray. In a large skillet over medium heat, heat oil. Add onion and cook until soft, 5 minutes, then add garlic and cook until fragrant, 1 minute more. Add ground beef and cook, breaking up meat with a wooden spoon, until no longer pink, about 6 minutes. Drain fat. Return beef mixture to skillet and stir in mustard and Worcestershire. Season with salt and pepper. Place bottom halves of slider buns in prepared baking dish and top with cooked beef mixture, cheese, tomatoes, pickles, and slider bun tops. Brush melted butter on top of buns and sprinkle with garlic powder and sesame seeds. Bake until cheese is melty and buns are golden, 10 to 12 minutes.