This delicate, mint-scented soup is one of the first recipes we return to when spring arrives in Minnesota each year. I have demonstrated it numerous times at local farmers markets and in front of a camera for our local television broadcast affiliates. It never fails to draw praise from those who have tried it at home.
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.
When the vegetables have drained sufficiently, coarsely chop them into 1-inch pieces and place in a high-speed blender or food processor, working in batches if necessary. Add the mint and court-bouillon and purée until smooth. Season to taste with salt and white pepper.
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.