Cut off the top 1/5 of the garlic bulb. Wrap in foil and Drizzle with oil. Roast for 30mins at 180C. This can be done in advance - store in a sealed container in the fridge until ready to use.
When garlic is cool, squeeze out the cloves.
Add garlic to a large saucepan with the butter. Saute for 2mins. add the garlic powder and the cream. Bring to a simmer. Add a good grating of nutmeg, and season to taste. Add more garlic powder if needed.
Meanwhile, Cut potatoes into thin slices using a food processor or mandoline. Try to keep the slices together as potato units - this will help with arranging them in the baking tray later. Peeling them is optional. If preparing them in advance, store in cold water.
Preheat the oven to 180C.
Arrange the potato slices vertically in the baking tray. Use a tray that lets them stay reasonably snug and vertical.
Pour over the cream and top with the cheese. Cover with foil.
Put potatoes in the middle rack for 1hour. Check progress - they should be mostly cooked through. At this point, you can remove foil and finish off to crisp the top, or leave covered and move to the bottom shelf until the rest of the dinner is ready.