1. Halve the leek lengthwise and cut into ½-inch lengths. Soak (see Note). In a large saucepan, combine the leek with the broth, carrots, parsnips, turnips, garlic, thyme, and bay leaf. Bring to a boil over high heat;reduce the heat to low and simmer until the vegetables are tender, 15 to 18 minutes.

2. Meanwhile, heat the oil in a large skillet over high heat until hot. Reduce the heat to medium-high and add the beef seasoned with salt and pepper. Sauté until well browned on both sides and medium-rare in the center, 8 to 10 minutes total. Remove the beef to a plate; cover loosely with aluminum foil and let rest 5 minutes, then slice thinly against the grain.

3. Stir together the sour cream, any juices from the beef plate, and the wasabi; add salt and pepper to taste. Discard the thyme sprig and bay leaf; divide the vegetables and broth among 4 soup bowls. Arrange one-fourth of the beef in the center of each bowl and top each portion with a spoonful of the wasabi cream.