Place cucumber, bread, water, vinegar, fish sauce, mirin and dill in blender; cover. Blend until coarsely pureed. Cover. Refrigerate 1 hour or until chilled.
Mix watermelon, onion, oil and mint in small bowl until well blended. Pour gazpacho into shallow bowls. Place a spoonful of watermelon mixture in center of gazpacho. Top with a dollop of crème fraîche. Serve with freshly ground black pepper.