01 - Combine sour cream, prepared horseradish, Dijon mustard, lemon juice, and a pinch of salt and pepper in a small bowl; stir until smooth. Set aside.
02 - Season beef with kosher salt and black pepper. Heat olive oil in a skillet over medium-high heat. Sear beef for 2 to 3 minutes per side until medium-rare. Transfer to a plate and let rest for 5 minutes, then slice thinly against the grain.
03 - Preheat broiler. Remove crusts from bread slices and cut each into two diagonal triangles. Brush both sides with softened unsalted butter.
04 - Arrange bread triangles on a baking sheet and broil for 1 to 2 minutes per side, until golden brown and crisp, watching closely to prevent burning.
05 - Spread a thin layer of horseradish cream on each toast point. Top with sliced beef, add a dollop more cream, and sprinkle with chopped chives. Optionally, finish with flaky sea salt.
06 - Serve immediately while toast points are warm and crisp.