1. Cut onion and fennel into cubes. Keep fennel leaves for garnishing soup. Peel beets and cut into cubes.
2. Heat oil in a large saucepan set over medium-high heat. Add onion and fennel. Cook for 5 minutes, stirring occasionally.
3. Add beets and pour in broth. Season generously with pepper and add a pinch of salt.
4. Bring to a boil over high heat.
5. Reduce to medium heat and simmer for 35 minutes or until a knife tip can pierce the beets.
6. Using an immersion blender or food processor, purée soup until smooth. Add balsamic vinegar and mix to combine.
7. Divide among 4 bowls. Draw a heart on top of soup with yogurt, if desired. Garnish with fennel leaves.

Keeps 3 days in the refrigerator or 3 months in the freezer.