Preheat oven to 170°C (fan forced). Grease a 2L (8 cup) capacity ovenproof baking dish with butter.
Place garlic, flour, mustard and curry powder in a bowl. Add the stock and stir until smooth. Add the sour cream and stir to combine.
Arrange half the potato slices over the base of the prepared dish. Sprinkle with half the onion and half the cheese. Season. Pour over half the cream mixture. Continue layering with the remaining potato slices, onion, and cream mixture. Reserve remaining cheese.
Cover with foil and place on a baking tray. Bake for 1 hour or until potato is tender. Scatter reserved cheese over the potato and bake, uncovered, for a further 25-30 minutes or until golden brown and the potato is very tender.
Set aside to cool slightly for 15 minutes before serving.