Preheat oven to 375F. Butter the sides of a heavy-duty popover or muffin pan.
Scrub and peel potatoes if desired, we peel or sweets and leave the skin on other varieties. Slice the potatoes and onion into thin slices on a mandoline (please use the guard to avoid serious injury). You can slice by hand but thickness will vary and may affect the consistency of the dish.
Heat the cream, crushed garlic cloves, nutmeg, Kosher sea salt and freshly cracked black pepper in a medium saucepan over low heat for 5 minutes.
While the cream mixture warms, layer the potatoes. Alternating, potato, onion, Parmesan, and Rosemary and so on and so forth until the tins are filled equally. Remove the cream from the heat and stir, spoon 1 Tablespoon of cream mixture onto each of the potato stacks.
Cover with heavy-duty aluminum foil and bake for 35 minutes. Remove foil and allow to brown another 7-10 minutes. Alternately you can turn the broiler on for 1-2 minutes to crisp the top. Or use a butane torch to fire it until browned and crisp.
Remove from the oven and allow to sit for 5 minutes before plating and serving.