Pommes Anna was created by a chef named Adolphe Dugléré in the 19th century in Paris. However, it was Julia Child that introduced the recipe to the American common folk and has been on record of calling it the "supreme potato dish of all time."
To make clarified butter, heat butter in a sauce pan low and slow for about 30 mins. As foam starts to form on top, skim it off and discard. Transfer the clear golden butter to another pan or container, leaving the sediment on the bottom of the pan to discard.
Chop the rosemary and set aside.
Peel the potatoes. Using a mandolin, carefully slice your potatoes to about 3/16 to 1/8 inch thick slices.
Once the potatoes are all completely sliced, set your iron skillet on low heat. Ladle some clarified butter on the pan and assemble the first layer of potatoes starting in the middle and spiraling all the way around to the edge. Add salt, pepper, and rosemary to the potato layer. Ladle some more butter and start another layer beginning at the middle this time going counterclockwise all the way around like the first layer. Repeat so on and so forth clockwise, counter clockwise, butter, salt, pepper, potatoes to about five layers or until all of your potatoes are used or until you come up to the ridge of your pan.
Place pan in an oven that was preheated to 400 degrees. You may want to put the pan on top of a sheet pan before placing in the oven, just in case the butter bubbles over during the cooking process. Cook for 30 minutes, then remove from oven. Drain the butter from the pan using a form fitting lid into a heatproof container. You may repurpose this butter for other recipes. Jiggling the pan, use a spatula to check if the potatoes have cooked away from the pan and check for color. Return the pan to the oven and cook for color for about 10 to 15 minutes. Outside crust should be golden brown and crispy.
When the potatoes are done, pull them out of the oven. Run your spatula around the edge to free any stuck potatoes. Invert a large plate over the top of the pan and carefully flip it upside down in one motion. Garnish as desired. Slice and serve.