Bring the vegetable stock to a boil in a medium-sized soup pot. Meanwhile, scrub the beets, peel off any rough edges, (it's OK if you leave on a little bit of skin), and remove the tops. Chop the beets into roughly 1- to 2-inch cubes.

Add the beets to the stock, return to a boil, and then lower the heat to a simmer. Cover and continue simmering for about 45 minutes or until the beets are soft.

Let the beet mixture cool slightly, then puree in a food processor (or using an immersion blender) until smooth. Add the yogurt and salt and continue processing until evenly pink. Chill soup until serving. Garnish with additional yogurt and rosemary if desired.