Preheat oven to 375 degrees.
Butter a 2 quart baking dish and then rub with the halved garlic clove.
Fill a large mixing bowl half way with cold water and set nearby. Grate the peeled potatoes using a regular cheese grater and place the grated potatoes in the cold water as they pile up. (If you let the potatoes sit out they will turn brown/gray.)
Once all your potatoes are shredded and in the water, swirl them around and then use a colander or mesh strainer (just something with holes small enough that the potatoes won't slip through) and drain off the water. Fill with cold water again and repeat this process 2 more times (you are rinsing off starch which helps keep the potatoes white). After draining potatoes for the last time, empty half of them onto a clean cloth and then ring out as much water as you can. Repeat with the remaining potatoes. You should have about 6-7 cups of shredded potatoes total.
Spread 2 cups of potatoes in the bottom of the casserole dish and then sprinkle all over with the onion salt. Repeat until you have used all the potatoes. Pour the cream all over the potatoes and then sprinkle the top with pepper. Cover the dish with a lid or tightly with foil. Bake for 40-50 minutes or until potatoes are tender.