Put the potatoes and a pinch of salt in a saucepan with plenty of cold water. Cover, bring to the boil then cook for 25 minutes until tender.
Meanwhile, put the fish and a pinch of salt in a large frying pan with just enough cold water to cover. Bring to the boil then reduce the heat immediately. Cover and simmer for 3 minutes. Leave the fish to cool in the liquid then drain and coarsely crush the flesh with a fork, removing any bones.
Heat 10g of the butter in a small saucepan, add the shallots then cook gently over a low heat for 5 minutes. Add the cream and remove from the heat.
Preheat the oven to 200 degrees C. Use 10g of the butter to grease four 300ml ramekins.
Drain and peel the potatoes then mash with a fork. Add the remaining butter, the shallot-cream mixture and some pepper. Beat in the egg yolks and mustard. Fold in the fish.
Add a pinch of salt to the egg whites, whisk until stiff then fold into the potato-fish mixture. Pour into the souffle dishes and smooth the surface with a spatula. Bake for 15 minutes until puffed and golden. Serve immediately.