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1
Put the potatoes into a pot, cover with salted water and cook until fork tender, 45 to 50 minutes. Drain potatoes and set aside to let cool slightly.
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2
Meanwhile, melt the butter in an 8-inch nonstick or cast iron skillet over medium heat, add onions and cook until translucent and tender, 6 to 8 minutes.
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3
Peel the still-warm potatoes and crush them gently into small pieces with a fork in a medium mixing bowl. Add the onions, cheese, salt, and nutmeg to the bowl and mix gently; do not overmix.
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4
Wipe out skillet. If using nonstick skillet, heat 1 tablespoon of the oil in the pan until hot. If using cast iron, heat the pan until hot but not smoking, then add 1 tablespoon oil. Add the potato mixture in an even layer. Let the mixture cook on one side until golden brown, 3 to 4 minutes. Run a spatula or knife around the edge and underneath the frico to loosen and give the skillet a shake to loosen frico from the bottom of the pan. Place a plate over the pan and quickly and carefully invert the frico onto the plate. Using a spatula, push the frico, browned side up, back into the skillet and smooth top if necessary. Let the frico brown on the bottom side, another 3 to 4 minutes. Loosen sides and bottom of frico again and slide onto a serving plate or cutting board. Set aside to cool slightly and set up. The frico will firm up as it cools. Cut into wedges.
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5
Mix together the remaining oil, cilantro, shallots and sherry vinegar, and salt to taste, Serve with frico.
