How to Make It
In a large saucepan, melt the butter. Add the onion and cook over moderate heat until softened, about 8 minutes. Add all but 2 tablespoons of the peas and the mint and cook for 1 minute. Add the stock and simmer for 5 minutes.
Working in batches, puree the soup in a blender until silky smooth. Return the soup to the saucepan and season with salt. Gently rewarm the soup and ladle it into 4 shallow bowls. Garnish with the reserved peas, the smoked salmon and a dollop of crème fraîche and serve.