1. Melt the butter in a microwave casserole dish for 1 minute on High.
2. Layer half the sliced potatoes in the base of the dish and season with salt, pepper and nutmeg. Cover with sliced onion, then top with the remaining potatoes and season again.
3. Cover and cook on High for 9 minutes. Remove from the microwave.
4. Whisk the cream and egg together and pour evenly over the potatoes making sure it runs down the sides. Cover and cook on high for 5 minutes.
5. Remove the cover and sprinkle the grated cheese on top. Microwave for a further 2 minutes or place under a hot grill to melt and brown the cheese.
based on a 600W oven