01 - Combine sour cream, prepared horseradish, Dijon mustard, lemon juice, and a pinch of salt and black pepper in a small bowl. Stir until smooth and set aside.
02 - Season beef tenderloin 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. Transfer to a plate and let rest for 5 minutes before slicing thinly against the grain.
03 - Preheat the broiler. Remove crusts from bread slices and cut each diagonally into two triangles. Brush both sides evenly with softened butter.
04 - Arrange bread triangles on a baking sheet and broil for 1 to 2 minutes per side until golden and crisp, watching carefully to prevent burning.
05 - Spread a small amount of horseradish cream on each toast triangle. Top with sliced beef, add a dollop of horseradish cream, and sprinkle chopped chives. Optionally, finish with flaky sea salt.
06 - Serve immediately while the toast points remain warm and crisp.