Preheat the oven to 350°F (175°C).
In a bowl, mix mayo, Dijon mustard, and horseradish sauce.
Slice slider buns in half and lay the bottom halves on a baking sheet.
Layer roast beef evenly over the buns.
Add a slice of provolone cheese on top of the beef.
Spread mayo mixture on the top halves of the buns.
Place top halves over the cheese.
Brush the tops with olive oil and sprinkle with salt and pepper.
Bake in the preheated oven for 10 minutes until cheese is melted.
Garnish with fresh parsley, slice, and serve warm.