In a large, heavy pot, heat the olive oil over medium heat. Add the onions, garlic, 1 teaspoon of the salt, and the white pepper. Cook, stirring, for 2 minutes. Add the cucumbers and chicken broth and bring to a boil. Reduce the heat to medium-low and simmer for 30 minutes.

Remove from the heat and stir in the parsley leaves. Let cool completely.

Process the soup, in batches, in a food processor or blender until smooth. Strain through a fine-mesh sieve. Cover and chill for 2 hours.

Meanwhile, in a small bowl, combine the tomatoes, red onions, diced cucumbers, chervil, the remaining 1/4 teaspoon salt, the black pepper, and extra-virgin olive oil. Stir to mix. Cover and chill until ready to use.

When ready to serve, add the yogurt to the soup and stir to mix. Ladle the soup into soup bowls and garnish with the tomato relish.