Heat the oil in a large saucepan. Steam the onions until translucent. Add the marjoram. Deglaze with 2 liters of water. Stir in the stock and add the split peas. Bring everything to the boil and simmer covered for approx. 50 minutes.
Remove 2 tablespoons of peas from the broth and set aside. Puree the rest of the split peas in the broth. Stir in frozen peas and 150 g cream and simmer for approx. 5 minutes.
Cut salmon into strips. With the peas set aside, add to the soup and season with salt and pepper as needed.
Lightly whip 50 g of cream. Serve the soup with cream and possibly fresh marjoram.