01 - Set the oven to 400°F (200°C) to prepare for toasting the bread.
02 - Trim crusts from bread slices and cut each slice diagonally to form two triangles.
03 - Brush softened butter on both sides of the toast triangles, place on a baking sheet, and bake for 5–7 minutes, flipping once, until golden and crisp. Remove and set aside.
04 - Combine sour cream, prepared horseradish, Dijon mustard, lemon juice, salt, and pepper in a small bowl. Chill until needed.
05 - Pat beef slices dry and season evenly with kosher salt and black pepper.
06 - Heat olive oil in a skillet over high heat and sear beef slices for 30–45 seconds per side until browned outside but still pink inside. Remove from heat and rest for 2 minutes.
07 - Spread a thin layer of horseradish cream on each toast point, arrange seared beef on top, add an additional dollop of cream, and garnish with chives and optional watercress or arugula leaves.
08 - Present immediately while toast remains crisp to ensure optimal texture and flavor.