Beet and Fennel Soup with Kefir

INGREDIENTS

• 2 tablespoons olive oil
• 1 cup chopped onion
• 1 cup chopped fennel bulb
• 1 1/2 teaspoons fennel seeds
• 2 large (2 1/2- to 3-inch-diameter) beets, peeled, cut into 1/2-inch cubes
• 2 cups low-salt chicken broth
• 1 cup unflavored kefir
• Additional unflavored kefir
• Fennel fronds (for garnish)