Layer 1/3 of potato slices in the bottom of the casserole dish, overlapping slightly, so that the dish is completely covered. Dot the top of the potatoes with 4 tablespoons of the butter, then season with 1/2 teaspoon of salt, 1/2 teaspoon pepper, 1/2 teaspoon rosemary. Repeat these layers two more times. Pour the milk over the top of the potatoes.
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