Bring a large pot of salted water to the boil and set up an ice water bath. Plunge the asparagus and spring onions into the boiling water and blanch until tender, about 3 minutes. Drain and transfer to the ice water bath just until chilled. Remove and drain on paper towels.
In a small nonreactive bowl, whisk together the sour cream and celery seed, and season with salt and pepper. Cover and refrigerate.
For a richer version of this soup, slowly add the cream with the blender still running until well incorporated. To serve, divide evenly among serving bowls. Drizzle a little celery seed sour cream on top and garnish with the hazelnuts and chives.