Melt the butter in a large saucepan and fry the onion for 5 minutes until softened. Add the stock and bring to a gentle simmer. Lower the fish into the pan and cook gently for 5 minutes until the fish has turned opaque. Lift the fish out with a slotted spoon on to a plate.
Add the peas to the saucepan and bring to the boil. Reduce the heat to its lowest setting, cover and cook for 15 minutes. Ladle about half the soup into a food processor or blender and process until smooth. Return to the pan.
Flake the salmon into small pieces and add to the pan with the dill and crème fraîche. Heat through gently and season to taste with salt and pepper.
Ladle the soup into serving bowls and spoon a little of the crème fraîche on top of each. Serve scattered with extra dill.