Peel the potatoes and chop roughly.
Place in a pan of salted water and cook until they are are soft.
Whilst waiting for the potatoes to cook, cook the fish according to the instructions on the packet.
Mash the potato, season and stir in the chopped chives.
Put the spinach in the bottom of an ovenproof dish, then place the cooked fish and sauce on top.
Pipe the potato around the outside, leaving a gap in the middle.
Sprinkle the cheese in the gap and top with the chopped spring onion.
Then either place under the grill or in a hot oven until the cheese is melted and the potato is golden brown.