Let the steak sit out a room temperature for 20-30 minutes. Season with salt and black pepper.
Pre-heat oven to broil. Heat up a cast iron skillet over medium-high heat, about 4-5 minutes.
Add 2 tablespoons olive oil to the hot skillet, carefully add the steak. Cook for 3-5 minutes on each side, or until desired doneness. Transfer the steak to a wood cutting board and cover with foil. Let the steak rest for 10 minutes before slicing it into thin slices.
Meanwhile, make the garlic butter and horseradish sauce. For the garlic butter, combine all the butter ingredients until well combined. For the horseradish sauce, combine all the sauce ingredients until well combined. Set butter and sauce aside until ready to use.
Drizzle both sides of the bread with remaining 2 tablespoons olive oil. Place the bread on a baking sheet and place in the oven under the broiler. Toast for 1 to 2 minutes per side, keeping an eye at all times to make sure bread does not burn. Top each crostini with a slice of steak, brush or drizzle steak with garlic butter and add a dollop of horseradish sauce. Top with cracked black pepper and fresh chives. Serve immediately. Enjoy!