Heat the heavy cream with crushed garlic and rosemary over medium until just starting to boil, then take off the heat and let steep for 20 minutes. Strain the cream through a sieve, then add salt and pepper.
Wash, peel, and mandoline the potatoes, then add to the cream and mix until coated.
Butter a baking dish and sprinkle the bottom with salt. Layer the potatoes into the pan and pour the leftover cream onto the potatoes. Dot the top with butter.
Bake, uncovered?, at 400°F for an hour.
