In a medium bowl, whisk together 1 cup sour cream, 2 tablespoons mayonnaise, 3 teaspoons prepared horseradish, 1 teaspoon Dijon mustard and 1 teaspoon fresh lemon juice until smooth.

Stir in 2 tablespoons finely chopped chives. Season with 1 teaspoon kosher salt and 1 teaspoon freshly ground black pepper. Taste and adjust: add more horseradish (½ tsp increments) for additional heat, or more lemon for brightness.

Cover and refrigerate for at least 30 minutes to allow flavors to meld. Can be made up to 24 hours ahead; stir before using.

Prepare and Cook Beef

Bring beef to temperature and season

Remove beef tenderloin medallions from the refrigerator 30 minutes before cooking so they come closer to room temperature for even cooking. Pat dry with paper towels. Season both sides of each medallion with kosher salt (approximately 1/6 tsp per medallion) and freshly ground black pepper.

Sear medallions

Heat a heavy skillet (cast iron preferred) over medium-high heat until very hot. Add 2 tablespoons olive oil and 1 tablespoon unsalted butter. When butter foams and oil shimmers, add the tenderloin medallions in a single layer without crowding (work in batches if necessary). Sear for 2–3 minutes on the first side without moving, until a brown crust forms.

Flip and cook the second side for 2–3 minutes for medium-rare (internal temperature 125–130°F / 52–54°C). For medium, cook 3–4 minutes per side (135°F / 57°C). Baste briefly with pan juices as they cook. Remove medallions to a plate and tent loosely with foil; let rest 5 minutes so juices redistribute (carryover will bring to final temp).

Toast Buns and Assemble

Toast buns

While the beef rests, preheat a second skillet or griddle over medium heat. Brush cut sides of 12 mini slider buns with 2 tablespoons melted butter. Place buns buttered-side down in the skillet and toast 1–2 minutes until golden and crisp. Remove and keep warm.

Assemble sliders

Spread about 1 tablespoon of horseradish cream sauce on the bottom half of each toasted bun. Place a small handful (about 1 tablespoon) of baby arugula over the sauce. Top arugula with a rested beef medallion. Spoon an additional 1 teaspoon of horseradish cream over the beef (or pipe if preferred). Add ½ teaspoon pickled red onions on top if using, and finish with microgreens or extra chives. Cap with the bun top.

Serve warm. These sliders are best eaten immediately so the beef stays juicy and the buns remain crisp.