Season a 1.5-pound beef top round roast with salt and black pepper.
Heat 1 tbsp olive oil in a skillet over medium-high heat until shimmering.
Sear the roast for 3–4 minutes on all sides to develop a golden crust.
Transfer the roast to a baking dish.
Bake at 350°F (175°C) for 1.5 hours.
Let the roast rest for 10 minutes before slicing thinly against the grain.
Toast slider buns lightly in a 350°F oven for 2–3 minutes.
Spread a smear of Dijon mustard on each bottom bun.
Layer 2–3 thin slices of roast beef on each bun.
Add shaved provolone cheese if desired.
Top with caramelized onions or arugula.
Place the top bun and press gently.
Drizzle with horseradish mayo or au jus if desired.
Serve immediately to prevent sogginess.