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1
In a steamer basket, steam the potatoes until tender, about 20 minutes.
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2
Let the potatoes cool slightly, then peel and cut them into 1/4 -inch-thick slices.
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3
Meanwhile, heat the canola and olive oils in a medium skillet.
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4
Add the onions and cook over moderately high heat, stirring occasionally, until softened and just beginning to brown, about 5 minutes.
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5
Reduce the heat to moderately low and cook, stirring, until caramelized, about 15 minutes; add 1 to 2 tablespoons of water if the skillet is dry.
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6
Stir in the garlic, rosemary, sage and thyme and season with salt and pepper.
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7
Preheat the oven to 350.
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8
Coat a 9-inch deep-dish pie pan with cooking spray.
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9
Arrange half of the potato slices in the pie pan, overlapping them slightly, and season with salt and pepper.
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10
Top with the onions and half of the Parmesan cheese.
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11
Cover with the remaining potato slices.
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12
In a bowl, combine the egg whites, whole eggs, skim milk, dry milk and nutmeg and season with salt and pepper.
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13
Pour the custard over the potatoes and sprinkle the remaining Parmesan on top.
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14
Bake the gratin for about 45 minutes, or until the custard is set and the top is golden.
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15
Let the gratin rest for 10 minutes before serving.
