Roast Beef Tenderloin
with Jellied Horseradish
4 3/4 teaspoons unflavored gelatin (from two 1/4-ounce envelopes)
1/4 cup cold water
4 1/2 cups sour cream
6 tablespoons drained bottled horseradish
6 tablespoons minced fresh flat-leaf parsley
2 teaspoons salt
1 (3 1/2-pound) trimmed and tied center-cut beef tenderloin roast, at room
temperature
1/3 cup coarsely ground black pepper
2 tablespoons vegetable oil
Optional garnish: watercress sprigs
Special equipment: an instant-read thermometer
Sprinkle gelatin over cold water in a small saucepan and let soften for 1 minute.
Stir together sour cream, horseradish, parsley, and 1 1/2 teaspoons salt in a large bowl. Add 1 cup sour cream mixture to gelatin mixture and heat over moderately low heat, stirring, until gelatin is melted; do not let boil. Add gelatin mixture to remaining sour cream mixture and stir well. Spoon horseradish cream into a 13-by-9 inch baking pan and refrigerate, covered, until firm, about 2 hours.
Put a rack in middle of oven and preheat oven to 350°F.
Pat tenderloin dry, then coat on all sides (not ends) with pepper and remaining 1/2 teaspoon salt. Heat oil in a flameproof roasting pan over high heat. Brown tenderloin on all sides (not ends), about 10 minutes total.
Transfer pan to oven and roast tenderloin until thermometer inserted diagonally 2 inches into center of meat registers 120°F, 25 to 30 minutes. Transfer beef to a cutting board and let cool to room temperature. (It will continue to cook, reaching 130°F for medium-rare.)
Remove string and discard. Cut tenderloin into 1/3-inch-thick slices. Cut horseradish cream into 2 1/2- by 1 1/2 -inch rectangles. Arrange tenderloin on a platter, alternating slices of meat with slices of horseradish cream (you will have leftovers). Garnish with watercress, if desired.