- Preheat oven to 200°C / 180 °C (fan-forced)
- Grease an 8-cup capacity (2 litre) baking dish with the butter.
- Finely chop garlic and sage together.
- Arrange a layer of white potato slices onto the base of the dish, overlapping slightly. Season with salt and pepper and sprinkle with a small amount of the chopped garlic, sage and cream. Add 2 tablespoons of the cheese over the potatoes.
- Next, top with a layer of sweet potato slices and continue layering with the garlic, sage, cream and cheese. Repeat layers of white potato and sweet potato until all the vegetables have been used.
- Top the final layer with remaining cheese and season again with salt and pepper.
- Bake 55-60 minutes or until potatoes are cooked and top is golden. Stand for 5-10 minutes before serving.
- If the top is browning too much while cooking, cover lightly with foil.