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Preheat oven to 375F.
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Spray a 2-quart casserole dish with cooking spray.
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In a medium saucepan over medium heat, sauté onions in 1 tablespoon butter and 1 tablespoon olive oil until translucent. Stir in cream, parsley, chives, rosemary, pepper, salt and nutmeg. Cook 2 minutes, stirring continuously. Remove from heat and set aside to cool.
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While cream mixture cools, combine sliced potatoes and milk in a large, microwave-safe bowl and cover with plastic wrap. Microwave at HIGH for 5 minutes, uncover and stir potatoes. Cover & microwave 5 more minutes. Drain potatoes & discard milk.
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Layer one-third potatoes, alternating between Yukon and Sweet, in the prepared dish.
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Pour 1/3 of cream mixture over potato layer and sprinkle with ½ cup Gruyere and 2 tablespoons Parmesan cheeses. Repeat layering the potatoes and cheeses twice. The final layer will be Parmesan cheese.
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If desired, dot 1 tablespoon butter into small pieces over the top.
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Cover with foil.
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Bake at 375F for 30 minutes. Uncover and bake 20-30 minutes more or until browned.
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Remove from oven and let set for 10 minutes before serving.
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Enjoy!