Pre-heat the oven to 190°C, 375°F, Gas Mark 5.
Lightly butter a large ovenproof dish. Layer the potatoes and onion, sprinkling with Garlic Salt and pepper between the layers. Pour over the stock, cover and cook for 40 minutes.
Sprinkle with cheese and cook uncovered for a further 35-40 minutes, until golden and potatoes are tender.