Step 1

Use extra-small red potatoes measuring less than 1 inch in diameter. Center-cut beef tenderloin roasts are sometimes sold as Châteaubriand.

Step 2

FOR THE BEEF AND POTATOES: Pat roast dry with paper towels. Combine 2¼ teaspoons salt, 1 teaspoon pepper, and baking soda in small bowl. Rub salt mixture evenly over roast and let stand for 1 hour. Tuck tail end and tie roast with kitchen twine at 1½-inch intervals. Adjust oven rack to middle position and heat oven to 425 degrees.

Step 3

Heat 2 tablespoons oil in large roasting pan over medium-high heat (over 2 burners, if possible) until shimmering. Add potatoes, scallions, garlic, paprika, 1 teaspoon salt, and ¼ teaspoon pepper and cook until scallions are softened, about 1 minute. Off heat, stir in water, scraping up any browned bits. Transfer roasting pan to oven and roast potatoes for 15 minutes.

Step 4

Brush remaining 1 tablespoon oil over surface of roast. Remove roasting pan from oven, stir potato mixture, and lay beef on top. Reduce oven temperature to 300 degrees. Return pan to oven and roast until beef registers 120 to 125 degrees (for medium-rare), 45 to 55 minutes, rotating roasting pan halfway through cooking.

Step 5

For the persillade relish While beef roasts, combine all ingredients in bowl.

Step 6

Remove pan from oven. Transfer roast to carving board, tent with aluminum foil, and let rest for 15 minutes. Cover potatoes in pan with foil to keep warm. Remove twine, slice into ½-inch-thick slices, and serve with potatoes and relish.