Melt the butter in a large saucepan set over a medium heat. Sweat the onion with a little salt for 3-4 minutes, stirring occasionally until soft. Add the potato and stock and bring to a simmer.
Cover the saucepan and cook for 15-20 minutes until the potato is tender. Once tender, transfer to a food processor and blitz with the cucumber, in batches if necessary, until smooth. Transfer back to a saucepan and add the cream and bring to a simmer before seasoning.
Meanwhile, place the haddock in a saucepan and cover with water. Bring to a simmer and cook the haddock for 2-3 minutes until cooked through, then drain. Ladle the soup into bowls and garnish with the haddock, chives and dill sprigs. Serve with sliced pieces of wholemeal bread.