Peel and chop the potatoes and boil until soft.
Push the cooked potatoes through a potato ricer or mash.
Add a little semi-skimmed milk to get a creamy mash consistency.
Make the sauce by mixing some of the cold milk with the cornflour to make a thick paste, then heat the milk to just below boiling point, about 3 minutes in the microwave or heat in a pan on the stove.
Pour the hot milk into the cornflour paste and stir until thoroughly mixed, then heat for 1 more minute in the microwave until thickened.
Season the sauce with salt & pepper or other seasonings of your choice. The sauce should be quite thick as the frozen fish and the spinach will release water while cooking in the pie.
Place the frozen fish in an ovenproof dish, add half the sauce, then cover with the spinach leaves.
Pour over the remaining sauce, the spinach will wilt with the heat of the sauce.
Top the fish and spinach with the mash (you don't have to pipe it, but it does look pretty).
Beat the egg and brush the top of the mash with beaten egg.
Bake for 40 minutes at 170C or until bubbling and golden brown.