Peel & mash potatoes well. Mix in cheese, yogurt, salt, and white pepper. Brush mustard in bottom of pie shell. Fill shell with 1/2 potato mixture. Layer on cheese and top with potato mixture. Dot with butter. Sprinkle with Parmesan. Bake at 375 F. for 45 minutes. Arrange tomato slices on top of pie 10 minutes before done. Cool 10 minutes.
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