Heat the oil in a pot over medium heat. Add the asparagus, onion and celery and cook until softened, about 5 minutes. Mix in the flour until well combined. Slowly mix in the stock, stirring constantly. Bring to a simmer and cook until asparagus is very tender, about 20 minutes. Puree the mixture in a food processor or blender. Return to the pot and bring back to a gentle simmer. Season with salt and pepper. Combine the sour cream and chives in a small bowl. Ladle the soup into bowls. Spoon a dollop of the sour cream mixture over top. Top it with smoked salmon and serve.