Melt the butter with the oil in a frying pan on the hob and fry the onions for 5 minutes. Stir in the garlic and fry for a further 2 minutes.
Layer up half the potatoes in a lightly greased ovenproof dish and scatter with the onions. Layer the remaining potatoes on top and season with salt and pepper.
Pour over the stock until it only just reaches the top layer of potatoes. Cover the dish with foil and place on the lower rack of the halogen oven.
Set the temperature to 250°C (482°F) and cook for 50 minutes or until the potatoes are cooked through. Remove the foil and bake for a further 15 minutes or until the potatoes are golden. Serve with roast beef, lamb or steak.