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