Preheat oven to 375 degrees F. Line a baking sheet with parchment or foil.
Separate biscuits and flatten each one to roughly 4 inches in diameter using your hand or a rolling pin.
Layer roast beef and one slice of cheese onto the center of each flattened biscuit. Spread a small amount of horseradish sauce over the cheese.
Fold the edges of each biscuit up and over the filling, pinching firmly to seal. Place seam-side down on the prepared baking sheet.
Bake 14 to 17 minutes until biscuits are deep golden brown and cooked through.
While sliders bake, make the au jus. Melt butter in a small saucepan over medium heat. Whisk in beef base, then add broth, Worcestershire, and garlic powder. Simmer 5 to 7 minutes until slightly reduced. Season with salt and pepper. Keep warm.
Remove sliders from oven and immediately brush with garlic butter finish. Serve warm with au jus for dipping.