For the cheesy mash topping, tip the potatoes into a large saucepan of boiling salted water and cook for about 15 minutes, until tender.
Drain very well, mash until smooth (don’t use a food processor – the mixture will be gluey), then return to the heat for a minute, stirring constantly, to dry out.
Add the butter and half the cheese, stir through, then beat in the crème fraîche. Season and put to one side until needed.
In making the sauce, melt the butter in a large frying pan and, as soon as it is bubbling, quickly add the pieces of fish so that they lie in a single layer.
Cook over high heat for just a minute or two on each side, so that the fish colour is very pale and not at all browned. Remove from the pan and set aside.
Add the flour to the buttery juices in the pan and cook over gentle heat for two to three minutes, stirring, to make a roux.
Add the onion, bay leaves, milk, stir, then cook gently for five minutes, stirring every so often, until thickened.
Add the fish pieces and stir gently into the sauce. Turn off the heat, then add the dill and half of the spinach.
Taste and season, bearing in mind that the cheese will be salty, then cover the pan with a lid or foil and leave to cool.
When cooled, stir in the remaining spinach, cover the pan and leave again for a few minutes.
To finish the pie, pile the fish with its sauce on a baking dish and the top is the cheesy mash.
Scatter the remaining cheddar over the top and bake in the preheated oven at 220C/425F/Gas 7 for 20 to 25 minutes, until golden and bubbling.
Brush the sauce on top of the pie with a pastry brush if it looks too thick.
Serving Suggestions: Serve with a squirt of lemon juice, accompanied by peas or green beans.