Peel the onions and cut into fine rings. Finely dice the garlic. Sweat both in oil until translucent. Wash the orange with hot water and dry it, rub off 2 teaspoons of the peel, add to the garlic and onion with the stock. Briefly bring to the boil.
Wash the fish, pat dry and cut into bite-sized pieces. Add to the soup and let it steep for 5 minutes. Season to taste with orange and lemon juice, salt and pepper. Sprinkle parsley on top before serving.