01 - Set the oven temperature to 400°F.
02 - Remove crusts from bread slices and cut each slice diagonally into two triangles.
03 - Brush both sides of bread triangles with softened butter, place on a baking sheet, and bake for 5–7 minutes, turning once, until golden and crisp. Remove and set aside.
04 - Combine sour cream, prepared horseradish, Dijon mustard, fresh lemon juice, salt, and pepper in a small bowl. Mix well and chill until served.
05 - Pat beef dry with paper towels, then season evenly with 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 but still pink inside. Remove from heat and let rest for 2 minutes.
07 - Spread a small amount of horseradish cream on each toast triangle, top with seared beef slices, add a dollop of horseradish cream, and garnish with chopped chives and watercress or arugula if desired.
08 - Present immediately while toast is still crisp.