Preheat the oven to 350℉ and line a baking sheet with aluminum foil.
Cut the sliders in half and place the bottom part in the baking pan.
In a small bowl, mix butter, whole grain mustard, Worcestershire sauce, onion and garlic powder.
Brush 2 tablespoons of the mixture on the sliders in the pan.
Top it with half of the roast beef and sliced cheese.
Top with the remaining roast beef and finish by placing the top part of the sliders on the roast beef.
Brush the remaining butter mixture on the sliders.
Cover with aluminum foil and bake for 12-15 minutes until the cheese is melted.
Uncover and bake for another 3 minutes. Serve immediately.