Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

In a large bowl, combine the ground beef, dry onion soup mix, Worcestershire sauce, garlic powder, salt, and pepper. Mix gently until just combined; avoid overmixing.

Press the meat mixture evenly into the prepared baking dish, forming a large, thin patty that matches the shape of the dish. Bake for 10 minutes until starting to firm up.

While the meat bakes, slice the stack of slider buns horizontally, keeping the tops and bottoms in one piece. Remove the meat from the oven. Place the cheese slices evenly over the cooked meat layer. Place the top half of the buns over the cheese.

In a small bowl, whisk together the melted butter, Dijon mustard, poppy seeds, and onion powder for the topping.

Brush the butter mixture generously over the top of the buns. Return the dish to the oven and bake for another 8-10 minutes, or until the cheese is fully melted and the tops are golden brown.

Remove from the oven and let rest for 5 minutes. Slice the large sheet of cheeseburger into 12 individual sliders using a sharp knife. Serve immediately.