1baguettesliced into 1/2-inch-thick (1 1/2cm) slices
2tablespoonsminced fresh chives
Season the steak with salt and pepper; let it come to room temperature.
Preheat the grill to 400 degrees F (205 degrees C). Clean and oil the grates.
Place the steak on the grill and close the lid. Cook for 6 minutes, turning halfway through. Flip the steak and cook until it reaches an internal temperature of 135 degrees F (60 degrees C), for medium-rare, about 6 additional minutes.
Remove the steak from the grill and let rest for 10 minutes before slicing into 1/4-inch-thick (1/2cm) pieces.
Melt the butter in a large skillet over medium heat. Add the mushrooms and cook until just golden brown, about 5 minutes. Season to taste with salt and pepper.
In a small bowl, combine the sour cream and horseradish until smooth. Season to taste with salt and pepper.
Preheat a broiler to medium-high heat.
Brush each slice of baguette with the oil and arrange them oil-side up on a baking sheet. Broil until golden brown, 1 to 2 minutes.
Spread each crostini with a little of the horseradish cream. Top with a slice of steak, a slice of mushroom, and sprinkle with the chives. Repeat with the remaining ingredients. Serve immediately.