03Roast 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.04For the crostini: Turn down the oven temperature to 400 degrees F.05Put 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.06For 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.07To 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.