01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - In a small bowl, combine the melted butter, minced garlic, chopped parsley, sea salt, and Parmesan cheese. Mix until well blended.
03 - Slice the slider buns in half. Brush the cut sides generously with the garlic butter mixture and arrange on the prepared baking sheet, cut side up. Sprinkle with additional Parmesan if desired.
04 - Bake the buns for 7 to 9 minutes, or until golden and fragrant. Remove from the oven and set aside.
05 - Pat the filet mignon medallions dry with paper towels. Season each side evenly with kosher salt and freshly ground black pepper.
06 - Heat olive oil in a heavy skillet or cast iron pan over medium-high heat until shimmering. Sear the filet medallions for 2 to 3 minutes per side for medium-rare, adjusting time to your preferred doneness. Transfer to a plate and let rest for 5 minutes.
07 - Place a few arugula leaves on each bottom bun. Top with a rested filet mignon medallion, a slice of Roma tomato, and a few slices of red onion. Drizzle with horseradish cream or aioli if using. Cap with the top bun.
08 - Serve the sliders immediately while warm. Pairs beautifully with a light red wine such as Pinot Noir.