Heat the butter in a non-stick pan. Add the onions and fry very gently for 15 mins until soft but not coloured. Stir in the beans, chicken stock, cream and a good pinch of saffron, then cover and cook gently for 5 mins.
Add the haddock fillets, then cover and cook 5-8 mins more until the fish is just cooked and flakes when tested.
Take out one-third of the haddock and set aside, then blitz the soup in the pan with a stick blender, or blend in a food processor until smooth. For a velvety texture, rub through a sieve. Tip into a pan or a freezer container, then stir in the remaining pinch of saffron and flake in the fish. If freezing, cover the surface of the soup with cling film before putting the lid on the container. To defrost, thaw in the fridge overnight.
To serve, reheat gently in a pan, ladle into bowls and scatter with the snipped chives.