Peel and chop the onion. Peel, wash and chop carrots and potatoes. Heat the fat in a saucepan. Fry onion, potatoes and carrots for 1-2 minutes and season with salt and pepper.
Add stock and simmer covered for about 30 minutes. Halve the orange, squeeze the juice. Remove 4 tablespoons of cooking cream. Pour remaining cooking cream and orange juice into the soup. Wash chives, shake dry and cut into small rolls.
Coarsely mash or puree the soup and season with salt and pepper. Arrange soup in deep plates and put 1 tablespoon of cream in the middle of each plate in streaks. Pluck the mackerel fillets and put them in the middle.
Sprinkle with chive rolls.