Preheat oven to 425 degrees F.
To make the dressing: Put mayonnaise, sour cream, ginger, garlic, sesame oil and rice vinegar in a blender or food processor mix until combined. Scrape down the sides of the blender or work bowl with a rubber spatula, mix for another 15 seconds. Transfer to a bowl, add the cilantro and mix well with whisk. Set aside.
Make the spice rub: Toast peppercorns and anise seeds in a dry large heavy skillet over moderate heat, stirring until fragrant, about 1 minute. Finely grind toasted mixture in an electric coffee/spice grinder or with a mortar and pestle. Sift mixture through a coarse sieve into a bowl and stir in salt, five-spice powder, and ginger.
Halve beef crosswise and sprinkle rub all over both pieces, pressing to adhere.
Heat 2 tablespoons oil in a large skillet with a metal handle over moderately high heat until just smoking, then brown 1 piece of beef on all sides, about 1 minute. Transfer to a large plate. Wipe skillet the clean with paper towels and brown remaining beef in same manner. Arrange beef pieces, 2 inches apart, in the skillet.
Roast in middle of oven 25 to 35 minutes, or until a thermometer inserted diagonally 2-inches into centers of each piece registers 120 degrees F. Let beef stand in pan on a rack for 25 minutes. Beef will continue to cook as it stands, reaching 130 degrees F (medium-rare). Discard string and slice beef.
Serve with dressing at room temperature.