01 - Combine sour cream, prepared horseradish, Dijon mustard, lemon juice, and a pinch of salt and pepper in a small bowl. Stir until smooth and 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. Remove from skillet and let rest for 5 minutes before slicing thinly against the grain.
03 - Preheat broiler. Remove crusts from bread slices and cut each diagonally into two triangles. Brush both sides of each triangle with softened butter.
04 - Arrange bread triangles on a baking sheet. Broil 1 to 2 minutes per side until golden and crisp, watching closely to prevent burning.
05 - Spread a layer of horseradish cream onto each toast triangle. Top with sliced beef, a dollop more horseradish cream, and sprinkle with chopped chives. Add flaky sea salt if desired.
06 - Serve immediately while toast points remain warm and crisp.