Heat the oil in a large heavy saucepan over medium-high heat. Add the onions, salt, and white pepper. Cook, stirring occasionally, until the onions are softened, about 4 minutes. Add the garlic and stir until fragrant, about 1 minute. Add the cucumbers, green onions, and dill. Cook until the cucumbers are starting to soften, about 3 minutes. Add the stock and bring to a boil. Reduce the heat to medium-low and simmer, stirring occasionally, for 20 minutes.
PurČe the soup with an immersion blender or in batches in a food processor or blender. Transfer the soup to a bowl. Cover and refrigerate until cold, at least 2 hours.
Stir the parsley and yogurt into the soup. Refrigerate for at least 2 more hours, or overnight, to chill thoroughly.
To serve, season the chilled soup to taste with additional salt and white pepper if needed. Ladle the soup into bowls and garnish each serving with a spoonful of chopped smoked salmon.
Yield: 8 servings