Melt the butter in a large pan then cook the onion and leek for 15-20 minutes or until really soft. Add the potatoes and cook in the butter for a minute, then pour in the stock and bring to a simmer. Cook until the potatoes are tender, then use a masher to roughly crush a third of the potatoes in the pan. Stir in the cream, haddock chunks and season with salt and white pepper. Bring back to a simmer for 2-3 minutes or until the fish is heated through, then stir in the chives and parsley before serving in warm bowls.