Preheat oven to 180°C/fan 160°C/gas mark 3. Lightly grease an ovenproof dish. Melt the butter in a saucepan, add the flour and cook for a few mins. Add the milk and whisk over a low heat until you have a thick sauce. The sauce will seem thick but that’s fine.
Wash the spinach leaves, chop into small pieces and squash down into the base of the ovenproof dish.
Thinly slice the tomatoes and scatter over the spinach.
Remove any skin and bones from the fish, cut into big chunks and put on top of the tomatoes.
Top with the sauce and sprinkle over the cheeses. Bake for 20-25 minutes, until the fish is cooked and the cheese is golden. Serve with the mashed potato.