6 cups low-sodium chicken broth

2 medium carrots (about 6 ounces), peeled and sliced 1/2 inch thick

2 medium parsnips (about 10 ounces), peeled and sliced 1/2 inch thick

2 small turnips (about 8 ounces), peeled, quartered and sliced 1/2 inch thick

1 medium leek, white part only, halved lengthwise, sliced 1/2 inch thick and rinsed well

2 cloves garlic, minced

1 sprig fresh thyme (or 1 teaspoon dried)

1 bay leaf (preferably Turkish)

2 teaspoons olive oil

1 pound flank steak

Kosher salt and ground black pepper

1/3 cup fat-free plain Greek yogurt

1 teaspoon prepared wasabi (can use horseradish or mustard)

Chopped fresh chives or parsley, to garnish