Add drained beans to slow cooker with white spring onion parts. Mix fish stock and mustard, lemon juice and salt and pepper together and then pour into slow cooker.

Arrange fish on top and sprinkle with extra pepper. Cover with lid and cook on low for 2 hours.

Remove the fish with a slotted spoon and place on a plate. Pour off nearly all the cooking liquid then mash the beans. Stir in the creme fraiche, the remaining onion and parsley. Taste to adjust with seasoning. Serve.