Peel and finely chop the onion. Fry onion and 3/4 of the peas in hot oil. Pour in the stock and cook over low heat for about 10 minutes. Puree with a hand blender and heat up again.
Stir in cream, add remaining peas and cook for another 5 minutes. Season to taste with cognac, salt and pepper. Cut salmon into pieces and pour over the soup with the chervil leaves.