Heat oven to 325. In a large saucepan or Dutch oven, combine potatoes and milk. Bring to a boil then reduce heat to low. Cover and simmer until potatoes just begin to get tender, about 3 minutes.
Place colander over large bowl and drain potatoes, reserving milk. There should be about 2 cups of thick starchy milk. If not, add enough milk to bring to 2 cups.
Rub 3 1/2 qt casserole dish with garlic and butter. Arrange sliced potatoes in two layers. Season with salt and pepper to taste. Dot with remaining butter and pour in the received milk and then the cream. Scatter grated cheese over the top.
Place casserole in the lower 1/3 of oven and bake until the cheese has turned deep golden brown and the milk has reduced and thickened, 80-90 minutes.
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