In small nonreactive bowl, whisk together the sour cream and celery seed, and season with salt and pepper to taste. 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 vegetable broth, and purée until smooth. Season to taste with 1/2 teaspoon salt and 1/4 teaspoon 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.