Preheat the oven at 375ᵒ. Grease a gratin dish well.
Add half of the potatoes slices by overlapping them and covering the bottom of individual gratin dish. Sprinkle generously with salt and pepper.
Add ¾ of the heavy cream and ¾ of the mozzarella cheese. Cover with the remaining potatoes. Sprinkle with salt and pepper. Add the heavy cream and the remaining of the mozzarella.
Place in the hot oven and cook for 30 to 40 minutes.