Use a pan with 3 inch walls roughly. Any oven pan will do, just make sure you can place the potatoes in it, covered in fat, with good margin.
Add the potatoes and cover them with the duck fat. Add a sprig of rosemary and cut the garlic cloves in half and add them to the duck fat.
Now fire up your grill or smoker, and have it running at 120-130° C / 250-265° F. If you're using the indoors oven, set it to the same temp.
Place the pan in the grill/smoker/oven, and let it sit there for two hours. If using a grill/smoker, maintain that same steady temp all the way.
When it's done, pour the duck fat in a container, it can be reused, and place the potatoes in a bowl. Sprinkle some sea salt on them, and serve. They can be served as a side dish, or as a starter. A cold beer is a great match!