Preheat oven to 375°. In a medium pan, combine whipping cream, 1 1/4 cups milk, garlic, salt, pepper, and cumin. Simmer (don't boil) over medium-low heat, stirring occasionally, for 5 minutes.
Meanwhile, peel potatoes and thinly slice crosswise. Spread level in a shallow 2 1/2-quart casserole or baking dish. Pour enough milk mixture over potatoes to barely cover. If potatoes aren't quite covered, add up to 1/2 cup more milk. Cover tightly with lid or foil.
Bake gratin for 30 minutes. Uncover and continue baking until potatoes are tender when pierced and top is nicely browned, 30 to 40 minutes longer.
Note: Nutritional analysis is per serving.