Preheat oven to 375°F or 190°C
Cut potatoes into large chunks and bring to the boil in salted water until soft but not mushy. Drain.
Cut fish into strips and drizzle with lemon juice.
Meanwhile sauté the onion, celery and carrot in olive oil until tender, set aside.
Melt the butter in a saucepan and stir in the flour until mixed.
Add the milk and seasonings, including parsley and stir continuously until close to boiling point.
Add the grated cheese and stir well.
Mix the fish and the onion mixture together in an ovenproof dish and then add the cheese sauce.
Tumble the cooked potatoes over the surface and spritz with olive oil to help browning.
Bake for about 35 minutes until golden brown.
Allow to cool for 10 minutes before serving.