Cut the potatoes into chunks and rinse.
Boil in plenty of salted water until tender.
Drain.
Heat 50ml of the milk with half of the butter.
When the butter has melted, mash the mixture into the potatoes until smooth.
Beat with a spoon, adding extra milk if needed, until light and fluffy.
Set aside but keep warm.
Half fill a pan with water and boil.
Add salt and the spinach.
Boil for one minutes, then drain and squash as much water out of the spinach as possible.
Keep warm.
Meanwhile, make the cheese sauce by melting the remaining butter in a pan over medium heat.
Stir in the flour and gradually add 250ml of milk, stirring continually to make sure you get a smooth sauce.
Once the mixture comes to a boil, lower the heat and leave to simmer for a few minutes.
Season with salt, pepper and mustard.
Add the cheddar cheese and stir until it is melted.
Dust the fish with flour, shaking off any extra.
Heat the oil in a frying pan and, when it is hot, lay out the fish.
Cook for 2-4 minutes a side, depending on how thick the fillet is.
To serve, divide the spinach between two warmed plates.
Place the fish on top and cover with the sauce, adding mashed potato on the side.
