Serve this delicate soup hot or chilled, in large bowls for a luncheon entrée or in soup cups or mugs as a first course.
Melt butter in large pot over medium heat. Stir in leeks and scallions, and cook until very soft, about 8 minutes, stirring often. Add potatoes, asparagus and stock, and increase heat to high. When soup reaches a simmer, reduce heat to low, and cook for 20 minutes.
Remove from heat, and let cool for about 5 minutes. Purée soup in batches in food processor or blender until very smooth. Return soup to clean soup pot.
To serve, reheat soup to a simmer just before ladling, and season to taste with salt and lemon juice. Ladle soup into bowls, and top with dollop of crème fraîche and sprinkle of chives and radishes.