• For the topping

1kg Maris Piper potato, cut into large chunks

2 large eggs, separated

400g frozen whole-leaf spinach (frozen weight), defrosted and squeezed dry

85g reduced-fat mature cheddar

whole nutmeg, for grating

    • For the filling

500g of pangasius fillet

1 onion, halved

a few whole cloves

1 fresh or dried bay leaf

300ml semi-skimmed milk

300ml fish stock (from a cube is fine)

25g butter

2 tbsp extra-virgin olive oil

50g plain flour

1 tsp wholegrain mustard

  1. Boil the potatoes for 20 mins until tender. Meanwhile, put the pangasius in a frying pan. Stud the onion with the cloves and put into the pan with the bay, milk, and stock. Bring to the boil, then cover and simmer for 2 mins. Bake the other side of the fish and cook for 3 mins more until the pangasius is just cooked through. Lift out the fish and set aside. Strain the cooking liquid.
  2. In another pan, melt the butter with the oil, stir in the flour and cook for 2 mins until it smells toasty. Take off the heat and slowly whisk in the fish cooking liquid. Return to the heat, whisk until the sauce boils and thickens, then stir in the mustard. Heat oven to 180C/160C fan/gas.
  3. Drain and mash the potatoes until smooth, then beat in the egg yolks, spinach, and most of the cheese. Season generously with salt, pepper, and nutmeg. Whisk the egg whites to soft peaks, then fold into the potato in 2 batches.
  4. Flake the pangasius into a large baking dish, pour over the sauce, and top with the spinachy potato. Scatter with the rest of the cheese and bake for 35-40 mins until golden and risen.