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