1.Place potato in a medium saucepan. Cover with cold water. Bring to the boil over moderate heat. Reduce heat; simmer for 15 minutes or until tender. Drain. Return to pan. Mash until smooth. Add cheese and half the butter; stir to combine. Preheat oven to 200°C (180°C fan forced).
2.Meanwhile, heat remaining butter in a large saucepan over moderate heat. Add leek, fennel and garlic. Cook and stir for 5-7 minutes or until vegetables soften. Add flour; stir to combine. Gradually add milk, stirring. Bring to the boil. Reduce heat; simmer for 5 minutes or until sauce thickens.
3.Add fish. Simmer for 5 minutes more or until just cooked. Remove from heat. Stir in cream, parsley and 1 tablespoon chopped, reserved fennel fronds. Spoon mixture into a deep, 2-litre (8-cup) ovenproof dish. Top with mashed potato. Brush with melted butter. Bake for 30 minutes or until lightly browned and heated.
Budget tip: Use celery instead of leeks. Floury potatoes will give the best results.Note