- 3 tablespoons extra-virgin olive oil
- 2 medium leeks, white and light-green parts, thinly sliced
- 1 large carrot, chopped
- 1 large stalk celery, finely chopped
- Two 28-ounce cans whole San Marzano tomatoes, crushed by hand
- 4 leafy stalks fresh basil
- Zest of 1 orange, removed with a vegetable peeler, and juice of 1 orange
- 1½ teaspoons kosher salt, plus more to taste
- ¼ teaspoon celery seeds
- Pinch crushed red pepper flakes
- 1½ cups plain Greek yogurt
- 1 English cucumber, peeled and chopped
In a medium Dutch oven or soup pot, heat the olive oil over medium heat. When the oil is hot, add the leeks, carrot, and celery, and cook until the leeks have wilted, about 6 minutes.
Add the tomatoes, slosh out the can with 1½ cup of water, and add that to the pot along with the basil, orange zest and juice, salt, celery seeds, and red pepper flakes. Set the lid ajar, and simmer until the soup is flavorful and the vegetables are very soft, about 35 to 40 minutes. Let cool slightly, then remove the basil and orange peel.
Purée the soup in batches in a blender with the yogurt and cucumber. (If a very silky-smooth soup is desired, you can strain the soup through a sieve after blending.) Transfer to a bowl or pitcher, and refrigerate until very cold, at least 3 hours. Taste, and season for salt again once the soup is chilled.