Pre-heat oven to 180°C: 350°F: Gas 4.
Melt 25g (1oz) of butter in a saucepan, sweat for 5 minutes, until softened.
Coat the fish with 25g (1oz) of seasoned flour.
Add to saucepan and cook for a further 3 minutes or until lightly browned on all sides.
Place the fish and onions into a buttered ovenproof dish.
Add 25g (1oz) butter and flour to the saucepan, cook gently for 1-2 minutes, stirring constantly.
Gradually whisk in the cider, lemon juice and anchovy essence.
Bring to the boil, whisking continuously, simmer for 2-3 minutes, or until thickened.
Pour the sauce over the fish.
Bake for 20 minutes.
Fry the apple rings in the remaining butter for 1-2 minutes or until lightly browned.
Drain the apple on kitchen paper and use to top the fish.
Place on top on the cooked dish, garnish with parsley.