Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the drained beans into the slow cooker pot with the white onion slices. Mix the fish stock with the mustard, lemon rind and juice and a little salt and pepper, then pour into the pot.
Arrange the fish on top and sprinkle with a little extra pepper. Cover with the lid and cook on low for 1½–2 hours or until the fish flakes easily when pressed in the centre with a knife.
Lift out the fish with a fish slice and transfer to a plate. Pour off nearly all the cooking liquid, then mash the beans roughly. Stir in the crème fraîche, the remaining onion and the parsley, watercress or rocket. Taste and adjust the seasoning, if needed. Spoon the mash on to plates and top with the fish.