Preheat oven to 350°. Arrange the potatoes on a baking sheet. Roast for 60-75 minutes, until very tender. Remove from oven to cool.. When cool enough to handle, peel the skins and discard them. Place the potatoes in a large bowl, mash until smooth. Cool completely. Stir in the milk, butter, vanilla, S&P, and egg yolks. Spread mixture evenly in a baking dish coated with cooking spray.

Whisk egg whites in a medium bowl until frothy. Stir in nuts, oats, brown sugar and cardamom. Sprinkle over the potato mixture. Bake uncovered, for 45-50 minutes, until top is toasted and edges are bubbling.