01 - Combine sour cream, prepared horseradish, Dijon mustard, lemon juice, salt, and pepper in a small bowl; mix thoroughly and set aside.
02 - Season beef with kosher salt and black pepper. Heat olive oil in a skillet over medium-high heat and sear beef for 2 to 3 minutes per side until medium-rare. Remove from heat, let rest for 5 minutes, then slice thinly against the grain.
03 - Preheat the broiler. Remove crusts from bread slices and cut each diagonally into two triangles. Brush both sides with softened unsalted butter.
04 - Arrange bread triangles on a baking sheet and broil 1 to 2 minutes per side until golden and crisp, monitoring closely to prevent burning.
05 - Spread a layer of horseradish cream on each toast point, top with sliced beef, add an additional dollop of horseradish cream, and sprinkle with chopped chives. Optionally, finish with flaky sea salt.
06 - Serve immediately while warm and crisp for optimal texture and flavor.