With a little advance preparation, the components for this hearty sandwich can be stored in the fridge and then assembled and served within minutes.
Prep: Preheat oven to 450 degrees. Place beef on a rimmed baking sheet. Rub with 1 teaspoon of the oil, and season generously with salt and pepper. Roast until an instant-read thermometer inserted in the thickest part of meat registers 135 degrees (for medium-rare), 30 to 35 minutes; let cool.
In a small container, combine sour cream and horseradish; season with salt and pepper. If preparing this recipe in advance, see storage instructions, below.
Serve: If stored in refrigerator, bring beef and horseradish cream to room temperature. To make salad, toss watercress with vinegar and remaining tablespoon oil; season with salt and pepper. Thinly slice beef, and place on each of four bread slices; top with horseradish cream. Serve with watercress salad.
Store: beef, covered, in refrigerator; horseradish cream, in a small container, in refrigerator; watercress, in a large bowl with paper towels, in refrigerator.