Heat oven to 375 F. Line a sheet pan with parchment.
Split each biscuit to make two rounds. Press each round flat with your fingers.
Spread a thin swipe of horseradish sauce on half of the rounds.
Top each sauced round with roast beef and a piece of provolone. Cap with the remaining rounds and pinch edges.
Brush tops with melted butter and sprinkle garlic powder and parsley.
Bake on the center rack until puffed and deep golden, about 15 to 18 minutes. Let rest 3 minutes.