Position a rack in the center of the oven and heat the oven to 375°F.
In a small bowl, combine the olive oil, rosemary, fennel seed, salt, and pepper. Stir to make a paste.
Pat the beef dry with paper towels and rub the paste all over the surface of the meat. If necessary, tie the roast at 1-1/2-inch intervals.
Put the roast on a rack on a small, rimmed baking sheet or in a shallow roasting pan. Roast until an instant-read thermometer inserted in the center reads 120° F for rare, 125° to 130 ° F for medium rare, or 135° F for medium, 40 to 50 minutes.
Meanwhile, in a small bowl, whisk together the crème fraîche, Dijon mustard, and lemon juice. Season lightly with salt to taste.
Transfer the roast to a carving board (preferably with a well for collecting juices) and let it rest, uncovered, for 10 to 15 minutes before carving it into 1/3- to 1/2-inch-thick slices. Serve the beef, passing the creme mustard sauce at the table.