For the beef tenderloin: Preheat the oven to 500 degrees F.
Place the beef tenderloin on a baking sheet, drizzle with the olive oil and season liberally with salt and pepper. Rub the oil and seasoning evenly over the meat.
Roast the tenderloin until an internal thermometer reads 125 to 130 degrees F for rare to medium rare, about 20 minutes. Remove from the oven, tent with aluminum foil and let rest until it reaches room temperature, at least 20 minutes. Transfer to a cutting board and thinly slice.
For the crostini: Turn down the oven temperature to 400 degrees F.
Put the baguette slices on a baking sheet in 1 layer. Drizzle with the olive oil and sprinkle with salt and pepper. Bake until golden brown and toasted, about 5 minutes.
For the horseradish sauce: Add the sour cream, applesauce, horseradish, mayonnaise, breadcrumbs and chives to a medium bowl and mix until combined. Season with salt and pepper.
To assemble: Spread a dollop of horseradish sauce onto each toasted baguette piece, add a slice of beef tenderloin and then top with another dollop of horseradish sauce and a few watercress leaves.