Peel the potatoes and cut into 1cm dice. Place in saucepan, cover with cold water and bring to the boil. Simmer for 12 to 15 minutes or until the potatoes are soft. Drain well and then add the butter and seasoning. Whisk or mash until smooth and lump-free.
Preheat the oven to 180'C.
Melt the butter over a medium heat and then add the flour. Whisk until well combined and then slowly add in the milk, mustard and nutmeg. Continue whisking until the mixture thickens and resembles double cream. Add the spinach and cook for a couple of minutes to wilt the leaves. Remove from the heat.
Bring a small saucepan of water to the boil, add the eggs and cook for 4 and a half minutes. Refresh in cold water, remove the shell and then cut into quarters.
Remove any skin and bones from the fish and cut into 1 inch pieces. Place in bottom of an ovenproof pie dish, add the eggs and season with salt and pepper. Pour the spinach sauce over the top, ensuring it is well distributed.
Allow the sauce to cool a little before spooning the mashed potato on top. Transfer to the oven and cook for 20 minutes. You want the top to be golden brown and the sauce to begin bubbling up around the edge of the pie dish.
«Brush the top of the pie with butter and score with a fork to get it nice and crispy.»