Preheat the oven to 230°C/450°F/gas 8.
Peel and dice the potatoes into 2.5cm chunks, then put into boiling salted water and bring back to the boil until ready for mashing.
Boil kettle and pour over the spinach to wilt it, then remove from the colander and gently squeeze out any excess moisture.
Drain the potatoes.
Peel and finely chop the onion, then peel, halve and finely chop the carrot.
In a separate pan, slowly fry the onion and carrot in a little oil for about 5 minutes.
Add the double cream and bring just to the boil.
Remove from the heat and finely grate in 2 good handfuls of cheese, then squeeze in the lemon juice and stir through the mustard.
Cut the fish into chunks then place in an earthenware dish with the spinach and mix together, pouring over the creamy vegetable sauce.
Drain and mash the cooked potatoes, adding a bit of oil, sea salt, black pepper and a touch of nutmeg (if using).
Spread the mashed potatoes on top of the fish mixture, spike it up to make it crispy. Optionally: add a small handful cheese on top.
Place in the oven for 25 to 30 minutes, or until the potatoes are golden.
Serve with some nice peas or greens.
