2. Add the potatoes, zucchini, and tomato paste. Just barely cover with water, turn the heat to high, and cover. Once brought to a strong simmer, turn the heat down to medium-low.
3. When the potatoes are very soft and the zucchini has given itself to the soup, about 20 minutes, turn off the heat. Add the lemon juice and cilantro, and season to taste with salt. Using an immersion blender, puree the soup to a smooth consistency; alternatively, working in batches, puree in a regular blender until smooth. The soup should shimmer in the ambient light. Serve hot or cold.
Sandor Katz lives to ferment; it’s his life’s work. The author of The Art of Fermentation shares how to make kombucha at home.