- 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.