Cut cucumber into smaller pieces and finely chop in a food processor. Reserve 2 tablespoons of chopped cucumbers for garnish.
Add yogurt, sour cream, lemon juice, garlic, olive oil, ground white pepper and sea salt. Pulse until smooth. If the soup is too thick, add some water and adjust the soup consistency according to your preference. Cover and refrigerate for at least 2 hours or overnight.
Just before serving, prepare the vegetables and make croutons to garnish the soup.
Pour the soup into bowls and garnish with finely chopped pepper, tomato, and cucumber and grated celery. Top with sizzling, crunchy croutons.